Just a few days in most cases, depending upon the extent of the tear.
Severe damage may take a fortnight but in either case, eat lots of vitamin C rich foods, a little protein with each meal and don't stretch the muscle, tight as it may feel. Leave that until it has healed fully.
If the damage is fairly superficial, try our Atrogel® as Arnica has long been known to speed tissue recovery, but if the tear is deep I'd advise adding some Arnica tablets to bring out the bruising.
One other natural remedy that is excellent for sort tissue injury is Bromelain (originally trialled very successfully on facial injuries in boxers!) but it is a bit pricey and you'll go through a pot within a week.
Keep active but don't overwork the muscle (eg: hill walking or cycling).