FMLA is 12 weeks per year. An employer has 3 ways they can calculate a year, and most choose rolling 12 months. So if your FMLA started July 1st, your 12 weeks won't start "refilling" until July 2021.

If your employer calculates one of the other 2 ways, your FMLA may reset sooner.

If your FMLA has been exhausted and you cannot return to work, you have to negotiate additional time with your employer. Your employer isn't obligated to provide more time off, though, and can let you go.

If your doctor thinks you just need another week or two (eg, your cast is coming off or stitches coming out) you can request a "reasonable accommodation" under the ADA. No guarantees on that, though.