15% of the fare for good service, 20%+ for exemplary.
If a limo service and you prepaid, figure 15 - 20% of the total fare as tip. Pay half on the drive in, half on the return.
I tend to over tip somewhat. We are eight and have a limo next week. I'll tip $20 each way, unless the driver turns out to be a jerk (which has yet to happen on any of our trips). Lots of times the driver provides some good trip insights and advice. Our total charge for the service is $120, so at 20% $24 total tip for both trips would be normal.