I do the Serving and returns EVERY time I play (4 times per week): 1 hour rallies, 45 minutes serving/returns, 15 minutes cool down
Those 45 minutes I serve like we play the point (1st/2nd), then we play the full point, not calling close outs, not keeping the score, playing relaxed. I estimate I get to serve about 60-80 times, usually 50 1st and 30 2nd. Then, the opponent serves, and I practice returns for 20 min.
Sometimes, we do 10 "points" each, then we switch, about 8-10 series each.
But we practice serving EVERY time.
Since I travel a lot, Serving is the thing that goes first, and only after 4-5 days...