Ewes come into season every 15-17 days, I think. Why don't you just mix the ewes and let him deal with them as one group? A tup should be able to cope with 40 ewes, if he's fit and in good condition.
I think most folk have the rams in for 4 - 6 weeks, giving the ewes 2 cycles to get pregnant. But it's up to you - if you only want a two week lambing period, put him in for two weeks. Downside is he might not successfully cover as many ewes as he would in a four week period. If you are using raddle, you'll be able to see when each ewe is mated.