A lot depends on what exactly are you going to do.
If you brush thin layers, you can start a layer of the next color right after the first layer get dry a bit - say, in 15 minutes. Obviously, you can't, say, put masking tape on a paint after only 15 minutes. It will simply lift the paint.
Mind you, I also do not recommend painting the second layer after 15 minutes. You can do it, but do you really need it? :)
I usually let acrylics to cure overnight before I'm starting painting different color, using masking tape, or after a thick layer of paint. However, theoretically, you can wait for 2 hours, if you don't mind a higher risk of lifting the paint and paing being less durable then as with proper curing.
In a perfect world, to achieve complete dryness, acrylic paints need at least 24 hours.
So, depending on what is more important to you, in my experience, variant are: 15 minutes - no masking, 2 hours - risky, 4+ hours - standard, 24+ hours - proper full dryness.
Experiment. :) Some people take shortcuts with their work; some don't. I do. :)