How much work time until fiberglass hardens?

Right of the bat, that seems like a pretty workable approach. My only concern is that you have two different binding agents: fiberglass resin vs. glue/contact cement. Glue/contact cement might dry out and age over time, weakening the bond between your material and the fiberglass reinforcement.

My approach is to rough up the material surface with some 120 grit sandpaper so that the fiberglass reinforcement has some more grooves, pits, and surface area to attach itself to. A smooth surface (like cast resin parts) won't 'grab' onto the fiberglass unless the surface has been roughed up somehow. Before reinforcing my MOW resin gauntlets with fiberglass on the insides, I used a coarse drywall sanding sponge to rough up the inside surface. It's never peeled away. Yet I was able to peel off big chunks of excess fiberglass resin that dripped on the smooth non-sanded outsides with my fingernail.

Yeah, Necronaut, I've been checking out your thread. cool stuff.

I would definetly rough it up first. And I would think that you would just need enough spray adhesive to position the cloth. Like I said before, I've never worked with fiberglass before but I would assume that the resin would soak through to the other side but I guess I really don't know that.

Everything I learned about laying fiberglass down I got from this guy's youtube channel: GasserGlass - YouTube
Most of his videos focus on making molds of car parts, but the basic principles and techniques still apply. Highly recommended!