About the same amount behind the CG as you'd put the main gear in front for a conventional gear.

Tim's response above is correct.  And I am not trying to diminish his answere in any way.  However, I offer the following:

I was trying to keep my comments brief, to reflect the number (exactly 1) of trike-geared planes that I have flown.  I was also hoping one of the more experienced guys would weigh in.

That approximate 15^o a forward of the aircraft CG (taking into consideration the true CG including its vertical position for a conventional (tail dragger) LG has been time proven for our models ever since before Bill Netzeband told us about it almost 50 years ago (April, 1957).  This also applies to full scale aviation.  This rule-of-thumb angle applies for models flying from a paved surface.  From grass fields, the gear can be mounted further forward.

As Tim mentioned, for a trike gear, that 15^o angle aft of the aircraft CG also works.  However, a bit of research, going back to Bill Netzeband's columns (and others), that rearward angle can be as small as 10^o (Netzeband again in March, 1971).  I have seen material written that for a stunt ship with a trike gear, the rearward position can be located just close enough to the CG such that the model will not set on its tail when at rest.  This will allow some really nice smooth non-bounce landings.  One thing to NOT do with a trike gear on a stunt ship is for it to give any positive angle of attack when setting/rolling on the surface.

More out of a combination of miscalculation and sloth than any deep thinking, I have my gear set at the "barely not stay settled on the tail when at rest position" (it started out too far forward and I tweaked it back just a bit).

The reason that I built my plane trike gear is because I fly on a field that has short grass and bumpy ground.  Good takeoffs and landings with conventional gear are very challenging, and considerably different than when landing on pavement.  With trike gear I can hold slight down on takeoff to mow some grass hold the plane to the ground until I'm ready for a smooth takeoff; with conventional gear I can do this on pavement but not my home field.  With trike gear as soon as my wheels touch on landing I can give it hard down without nosing over; with conventional gear I can give slight down to prevent bouncing, but on my home field even slight down while I still have airspeed results in a nose-over.

Now, I am going to beat a dead horse.  In spite of the insistence of some probably well intentioned but less experienced and technically challenged yet vocal contributors to these forums, the landing gear position for a stunt ship has nothing to do with its relationship to any portion of the wing leading edge.  The critical parameter to properly locate the landing gear is its position relative to CG of the model, regardless of the wing planform.

I will say this for the "so-and-so inches from the leading edge" criteria -- it gives a dimension that is understandable, and on similarly-sized airplanes of similar planform, it just about has to get you into the ballpark.  I considered giving that "fifteen degrees" criterion, but in the hours BC this morning (Before Caffeine), I just couldn't figure out how to make it sound clear -- Keith did a pretty good job of it.

The downside is that "so many inches in front of the LE" doesn't work if you're building something bigger or smaller, or with a wing that's higher or lower, or with a wing that's more or less tapered, etc., than the prototype plane for which the rule was made.  Projecting a line 15 degrees ahead or behind of "down" from the CG will get you the correct position of the LG every time.

OK, another trike gear question.  Does it matter if the strut is vertical, angled fwd, or angled aft so long as the wheels end up at the 15° point.  Seems like most of the trike designs I've seen the strut exits wing aft of the wheel and angles fwd.  I'm thinking it would be stronger / easier to use the spars as the attach point for top of strut which would require strut to angle to the rear.  Or is it mostly a aesthetics thing ?

Also, when measuring the 15° angle, is that to the wheel axle or where it contacts the surface ?

In my way of seeing the thing the 15 degrees applies only to taildraggers and has to do with the angle at which they it on the ground.
A tricycle gear is a whole different thing.  On a real airplane the main gear is so close to the CG that they tend to sit on the tail when being refueled or loaded.  The nose gear is a wimpy little thing that hardly takes any stress at all.  The pilot lands on the mains and gently elses down the nose gear as airspeed goes away. 

I built a Jetco Sabre with the gear in the kit position.  The mains were way too far aft.  The result was that upon landing the nose was slammed down, damaging the nose tire.  I beefed up the nose gear, but the correct fix would have been to move the main gear forward.

In my way of seeing the thing the 15 degrees applies only to taildraggers and has to do with the angle at which they it on the ground.
A tricycle gear is a whole different thing.  On a real airplane the main gear is so close to the CG that they tend to sit on the tail when being refueled or loaded. 

how odd, I just drove by the airport,, and nothing there but trike gear airplanes,, not one of them was sitting on the tail,, I am curious as to what "real " airplanes you are refereing to Paul,,,

all the references I have seen and my personal experience dictates as has been stated,, the mains touch the ground 15 degrees aft of the cg,, like everything else on a stunt ship though, some trim may be needed,,