Depends on the back surgery he needs. I've had 4. The first was a "laminectomy" which just cleaned out the "holes" where the nerves enter the spine. It's no big deal although they made me take 3 months off.
The other 3 were all fusions. T8-T11 was 2 rods and 8 screws that kept me out of a wheelchair and diaper for the rest of my life. I was fortunate they caught that one before it went because there were no symptoms. Wore a 2 piece shell for several months that had a belt around it and tightened it to support my torso. After 3 months I was able to go back to doing whatever, but probably not anything like playing football.
L3-L5 was a lower back fusion, 2 rods, 6 screws that really doesn't limit anything except I had to take 3 months off, again.
C6, C7. T1 fusion was a plate and 6 screws, with no time off needed and didn't slow me down at all.
So, it all depends. "Back Surgery" can include anything from minor, 1 day inpatient surgery, all the way to a week or so in the hospital, with months to recover only to be limited in what one can do.
With JJ Watt, he just needs to build his core back up and get back to playing speed.
When recovering from back surgery, it's impossible to maintain the weight that these guys carry, so they have to take the requisite time off for everything to heal, hope that scar tissue doesn't develop, then start working their way back up to playing weight and speed.
My guess, since he's been playing with the pain, and it hasn't gotten better, probably something that will take him out for this season, but if surgery and recovery are successful, he'll be back next year with very little limitation.