My first tunnel time was 4 minutes, I felt fine the day after. I'm now up to doing 10 minutes a session but I did feel it to begin with!
I did 10 minutes (5 x 2min flights) and an additional 8.5 minutes (2 x 2mins, 1 x 1 min and 1 x 3.5 min) on Tuesday and although I felt fine the next day I was getting tired in the tunnel. I'm learning to sitfly so my arms and back were giving up!