Jasmine is limping

Hi, Just a quick question. Jasmine has been limping slightly for a couple of
days.
Her paw isn't red or swollen, and the limp is only a little bit, so I'm
keeping an eye on her. How long should it take to heal, and when should I
take her to the vet if it doesn't? I've also caved in and given her the
expensive food again. She was eating a small amount of the cheaper stuff,
but I don't think it had enough nutrition in it anyway. And she kept raiding
the bin. I know, I'm a sucker.