How long should it take for my pony to lose some weight and get jumping and competing fit?

if she's been out of work because of an injury, it's going to take much longer to get her fit because you don't want to rush and injure her again. i work around a model of an 8 week fittening programme which you can change to suit your horse - ie whether they stopped working because of injury or not, how long they've been out of work, if they're fat, etc. seeing as your pony has been injured and it overweight i would say she won't be ready to compete in march. ask your vet for a suitable fittening programme that won't aggravate her injury or her joints due to her being overweight.