Well, if you're on an iPad 3 already, you can hold off until iPad 6 or 7 unless there's some urgency i.e. certain apps won't run on iPad 3 anymore.

My rule of thumb is to skip until they announce the device is EOL and won't receive anymore updates, and when my apps start dropping dead like flies sprayed with ridsect (i.e. Only reason I upgraded to iPad4 from iPad1 was because the Ashens app and Cut the Rope stopped becoming available for it).