My mum had cellulitis 12 weeks ago & recovered. she still gets a tingling feeling in her affected leg, is this normal and how long can it last for?

Yes be patient: 12 weeks seems like a long time and quite honestly it's possible that some degree of neuropathy may also last in her legs but the symptoms of paresthesias or tingling are as a result of the infection. Depending upon severity she may take up to a few more months or up to 1 year to resolve. Cellulits of the leg can be serous all structures affected can be altered for some time afterward.