If your instructor has a very conservative approach to ukemi, i.e. he or she has you sit in seiza and put your shoulder on the mat and look behind you and then roll from seiza, then trust your instructor. They are conservative enough that you won't get hurt.
However, some very good instructors (or sempai) may let YOU judge what YOU are ready for. They may teach ukemi to the entire class and ask if you feel ready to try it. Trust your instincts. Don't let your ego prevent you from saying no if your gut is telling you that you're not up to it. 5 years from now it won't matter whether you learned a technique at 2 months or at 6 months into your aikido journey. I learned this the hard way trying to learn breakfalls before I'd really learned rolls.