Depends how bad obviously, but probably months :(

According to UCSF Website:

Recovery times differ depending on the severity of the injury:

A minor, or grade 1, MCL tear can take from a few days to a week and a half to heal sufficiently for you to return to normal activities, including sports.
A grade 2 tear can take from two to four weeks to heal.
A grade 3 tear usually takes from four to eight weeks to heal, unless it is associated with damage to the ACL, in which case the recovery time may be longer.

While Elia looked in difficulty in the match, I think he was also back on the pitch at full time without crutches.p, so maybe at the less severe end of the scale?

He's been superb. A real revelation. Gussy said he could become the best ball-playing hooker in the Prem, with hard work. Looks like he's responded well.

On his Instagram, he describes it as a "Minor setback", so hopefully it's at the lower end of the scale...