Very often, epoxies are made for somewhat specialized purposes.

If you've got some 3 hour you might read it's label or enclosed literature to see what it was made to do specifically. You might find that it was for layups or glasscloth molding. Or as surfacing resin, also called finishing resin.

For our models, 15 and 30 minute are excellent. And are usually made to be glue for assembling structure. The 30 is really good for reinforcing cowls and such with glasscloth, as well as doing a great job of gluing firewall bracing and such, or joining wing halves.

30minute is often the best value for the expense. I just used up the last of some I bought a year and a half ago, and in that time have built over 15 ARFs. And repaired a number of them. And doubled about half the cowls in that bunch.

As you get experience using it, you'll get good at squeezing out just the amount you need. And won't be wasting any, and certainly not wasting the blobs that kick on you before you can get it all applied.