Some batteries will specify.  In the absence of that, no more than 1/10th of the rated discharge is a good rule of thumb.  I push this to 1/5 on some of my RC packs when I'm feeling impatient, for what good it does.

Keep in mind that the proper way to charge a lithium polymer cell is never, ever, let the cell voltage get above a critical value (4.2V springs to mind, but I've seen 4.3 -- I'd have to look it up to know for sure).  Chargers are designed with this in mind, and they won't let the cell voltage exceed the limit.  The way that you limit the voltage in a charging battery is by reducing the current until the voltage is OK -- so once the cell voltage has reached the limit with maximum current, the charger will start ramping the current down.

What this all means is that if you increase the charging current from 1C to 2C, you won't come close to reducing the charge time by a factor of two -- you'll only cut the current-limited phase by a factor of two, after which the voltage-limited operation will take the same amount of time either way.