I am working on revolver about once a week and trying to set some realistic goals to work towards.

For example, on a drill with six targets, one shot each, reload, repeat-
Or three targets two shot each, reload, repeat,
I was initially taking 7.5 seconds longer with the revolver than the semiauto,
And have gotten down to 3.5 seconds longer.

Is there a rule of thumb of how much a revolver reload should take compared to a mag reload?

Since there are multiple ways to reload a revolver I don't think anyone has said that it's always X seconds slower. The speedloaders that push to unload into the cylinder are a little faster than the speedloaders that you have to turn the knob, at least for me. Full moonclips are faster than half moonclips or any speedloader. Loading two cartridges at a time from the rubber strips are the slowest method that uses a mechanical assist, thumbing in the cartridges one at a time is slower still.
For an idea of how fast you really can reload a revolver when you are the worlds best take a look at Jerry Miculek. And know that he is just as exceptional with a semi-auto.

There was an interview where Miculek said he could shoot 6 shots with a revolver faster than he could with a semi-auto.  He said that he was slower with the semi-auto because he had to wait for the slide to cycle with the semi-auto, and with the revolver he controlled the speed of the cylinder's rotation, and hence the speed of the system.

Yeah, that guy is a whole other level beyond GM.

Especially considering he was basically Joe regular dude machinist who got into shooting when he was pushing 30,
Vs training from childhood like a Chris Tilley or something.

There is a ton of info on first shot, splits, reloads, etc. for semi auto, not so much for revolver.

A good USPSA reload for a revolver is under 3 seconds shot to shot. Real good is more like 2 seconds.
My best splits on a 625 are .23
My best splits on a 1911 are .13
Transitions and first shot should not differ between a revolver and semi (of similar recoil).

In the drill you mention, the only real difference in time should be the difference between your semi auto reload and revolver reload. I would think that should be under 2 seconds with some practice.

Thank you.
My splits are very close.
My transitions are lagging.
Not sure if it’s from driving the bigger gun, (I’ll try the 4inch one vs 6.5) or if just so much more time behind a semi auto vs a recolver.

My reloads were about 6 seconds initially  and dropped a little over 2 since.  That needs some more work.

Another thing to think about for a stage is that you're reloading more often as well.