I don't think I'd want to go much past 3-3.5 years. I think *if* I were to ever have another LO a 2.5-3 year age gap would be nice since LO1 and LO2 are only 19 months apart, it can be pretty tough sometimes having them SO close.
What is the largest age gap you would consider between your kids? Or does that not make a difference one way or another for you?