Would be boarder-line I'd imagine and depend a lot on her overall physical health. Older maiden mares are often very difficult to get in-foal as they tend to have delayed uterine clearance and retain fluid after breeding (so frozen is not usually recommended). Not impossible to get them in-foal, but they can require more intensive vet care than the young maidens or mares who have recently foaled. Also, coming into season every month throughout the year is not normal and may indicate something amiss with her hormones. Some mares do come into season year round, but I would certainly mention it to the vet and have a pre-breeding exam done. All this being considered, you could probably pick up a nice weanling for the same cost or less it would take to breed this particular mare. With her being an older maiden I would expect her to take a few cycles to catch. Either way, discuss it with your vet and if you are serious about breeding her, have a pre-breeding exam done and then go from there.