depends on the driver size, the driver's characteristics, and the slope of the crossover you're using. also depends on how loud you plan to turn it up, and what you are listening to.
for my 5.25"s i use 95Hz @ 18dB/octave - i don't listen super loud - if i did, i would consider maybe 120Hz, or a steeper crossover slope. on a previous system with 6.5"s, i've had success with settings as low as 80Hz.
for any given driver, if you are using a 12dB/octave slope, you'll want a higher cut-off point. steeper 24dB/octave slopes can allow you to go a little lower.
if you don't listen very loud, lower cut-offs around 80Hz can work fine (and sound better IMO), but if you really like to crank it, higher values up around 120-150Hz are the safe choice for long speaker life.
one way that has always worked for me is to put a variable or programmable active crossover ahead of your amp. play the system with the subs off and experiment with different cut-offs until you find the lowest setting that sounds good at all volumes with all types of music. then turn the subs back on and blend them to match up with the mids. good luck.