We're looking at getting a limo for 5-6 hours, costing $500 -600. 20% would be another $100-125, do you still tip 20% on such a large bill?
Tips and gratiuties are personal decisions, and I tip based on my satisfaction with the product/service. So, as a rule, I can't say I automatically tip x% - but, if the service was good, then I would have no issues tipping 20%.
I would also tip 20% for excellent service. The drivers rely on tips to earn a decent living.