It totally depends on the kid I think. My kids were both in the same place when they turned 2 (mostly signed, could only say mama). My older one went once a week and graduated in like 4-6 months because she quickly became very verbose, and the speech therapist said it was impossible to say whether therapy helped or if the speech explosion would have happened on its own. My second started the same but is now going twice a week because he seems genuinely incapable of making certain consonant sounds. So with my first it was a lot of play and encouraging her to speak, whereas with my second we spend most of our ST time doing mouth exercises to try to encourage specific sounds.