Hi, first, are you absolutely sure this cat is homeless? Do please take her to the vet to be scanned for a microchip (that shouldn't cost you anything) and put a "found" ad in the local paper (again, usually free)

Assuming you have done these and are sure she really is a stray, then the answers are as follows:

The norm for rehoming moggies is 8 weeks, please don't let them go any earlier than this. Pedigrees go at 13 weeks fully vaccinated which is best for everyone if you can do it but it is very unusual for moggies to get treated as well as that. Vaccinations are done at 9 and 12 weeks. One big advantage of keeping the kittens this long is that Mum is unlikely to come into call again while she is still feeding ktitens - and they will happily feed for as long as she will let them. Also it is unlikely that Mum will call in the Autumn, but I have known them call right at the end of December.

She needs to lose her milk before she is spayed, once you get to January you should certainly watch out for signs of calling especially if we have a warm spell (which does happen occasionally in January). The combination of warmer weather plus lengthening days is enough to set many girls calling. Obviously if there is any question of calling you must keep her in. The good news is that if there are any entire males around at all, they will know she is about to call before you do and act as a sort of early warning system so, if you are vigilant, you should avoid any accidents. But it does take vigilance. The safest option obviously is to confine the cat from the first warm spell after the end of December until she has been spayed. Another option (which is what I have done in the past when I had a girl I wanted spaying and needed to be sure she was not pregnant) is simply to ask your vet for Ovarid and give her 1/4 tablet every three weeks starting from when the kittens are about six weeks old until you get her spayed - you will only need one tablet.

Liz