In my experience, you can re-coat after about an hour, but you'll want to leave it overnight before you ride it. So you can probably do a couple coats without putting your bike out of commission in any real way.

I agree with mkeller - I prefer the feel of raw tape, but I've gotten used to shellac. I only put on 3 or 4 coats though.

I would imagine your fade/soil will even out and disappear with a couple coats of shellac. The treatment will certainly make the tape last longer.