depends on the mare, if she is young for her age or also whether she is still realy cycling and truely in season. the older they are the more difficult it is to get in foal and the more likely to have problems / complications with the pregnancy. it also depends on whether she has had a foal before. I would realy suggest talking to your vet who can do an MOT and check hormone levels/cycling or ovulation. Really think long and hard about it before going ahead - its a big commitment and not one to be decided on the spur of the moment.