It may be quicker by boat. It’s the first stop on a 5 minute boat ride from MK. The monorail stops at The Contemporary and the Polynesian before stopping at The Grand Floridian.
Generally, I think there is a 15 minute grace period at many of the Disney restaurants.
The boat ride is quicker, but monorails run more frequently. If it looks like a boat is heading back to MK, get on that. If it looks like it just left, take a monorail.
As the PP said, there is no official grace period. Disney recommends you show up 15 minutes prior to your ADR time. Technically, they can turn you away if you check in a minute past your ADR time (have seen it happen before).
The boat ride is quicker, but monorails run more frequently. If it looks like a boat is heading back to MK, get on that. If it looks like it just left, take a monorail.
As the PP said, there is no official grace period. Disney recommends you show up 15 minutes prior to your ADR time. Technically, they can turn you away if you check in a minute past your ADR time (have seen it happen before).
The boat is faster if it is close by, however as others have pointed out the monorail runs mre frequently. I would allow at least 30 minutes travel no matter which route you take. If the restaurant is very busy there is a chance your ADR will not be honored and you will be charged a no show fee.
Would agree with planning on 30 minutes, that gives you a little buffer.
Conside taking the boat over, its a nice ride and monorail back. That way you get to experience both.
I would probably make it 45 minutes instead of 30. There are three monorail stops (CR, TTC, Poly) plus sometimes there are afternoon maintenance checks that can add 10-15 minutes.
Another option, depending on the weather, is ferry boat or express monorail to TTC and walk from Poly to GF. We have walked from Poly to GF and back many times, and at a very leisurely pace it took 15 minutes.