I've been thinking about this lately too. My consideration is, if I leave them in, how long will the ewes need to gather their strength for the next pregnancy?
I tupped mine late October so lambs are due late March. If they are with mum for 5 months then she will have two months roughly to recover, at a good grassy time of year. Is that enough? Is it just a case of keeping an eye on their condition? By 5 months I guess the lambs are not getting a lot of milk in any case.
The ram lamb we used this year was 9-10 months old. How early are the boys sexually capable? 
The younger ram lamb I used this year was 6 months old the other one was 8 months old, both covered and got in lamb all of the ewes they were put to within the first ten days (although I used a teaser)
Granted my Charmoise ram lamb was only put to 7 charmoise ewes
This year I am lambing in March due to uni term times but I would usually lamb in April as I lamb outside and the weather is nicer at that time, so lambs would be born in April, they then would be weaned beginning of September so 4-5 months old, fist two weeks in september the ewes are put on poor grazing (scrub) to dry up, after that they are moved to better/good grazing which puts condition on and may flush a little before the ram goes in November (although I dont really want to flush the mules anyway)
The pedigree charmoise lambs are weaned at 4 months of age, wich is a pain this year as I have december/january born ones needing weaning in May which I have no idea where to put them yet

I bought the ewes in lamb at welshpool, wouldnt usually lamb this time, although the lambs are as hard as nails as although the ewes walked into an open field shelter to lamb them, they havnt been back in since so have been out in all the frost/snow from hours old