Sorry, little later, but better later then never :-P ... I was in Spain for some time.
so few comments:
Keith is right, I had timer on 8 pin 12F PIC controller which did not have any other elements only PIC and connector, worked well, but programming by ON/OFF switch was difficult to explain to some not-techie people (one of my Czech friends did official ECh flight in teaching mode), so that is why I decided throw it away and use JETIBOX ... and all were happy :- )) ... so I fullly agree with Keith, simple solution is may be atractive, but sonner or later ... :- P ... I think Will Hubin can have that my program, I think he sold it for some time. (and then happened what had to happen :-)) )
Yes, I have simple timer, it costs 15E (one piece - if you want more, we can speak about it), and needs programmer fro 35euro. However we use it for teaching kids, that programmer costs 2x price of timer, but you need only ONE for whole class, so no problem, and it is universal programmer for all Jeti devices (including ESCs) and it is also signal generator and signal monitor, so it is not lost money
Arduino is good thing for amatheur timers, beside that piece mentioned by Pat, there is even smaller arduino for 1.5$ or so but it is still enough for trivial program like timer. Programming is by USB and compiler is free, so such program can be delivered as source code and so values (delay, flight time and throttle) can be hard coded, so they can be easily programmed on PC by USB cable. I know, pretty dirty solution, but will work well. I remember Bob H. programming his timers on field by notebook, so what?
If you want really on firld programmable arduino, the simplest programmer we used was simple potentiometer - if you connect it to arduino, program can recognize it is connected and trigger learning mode, so you can set RPM automatically stored to EEPROM - then disconnect and fly .. simple :-P no special hardware needed ... the difference to Jetibox is obviouse, jetibox (or any other digital, easy to set) solution will allow store and recall tested values in case you use it on different models, or in different conditions etc, while pot wil not gove chance precisively set values, but it is still good enough for beginners.