Broad question because it depends on where and how you epoxy the weights, to what degree of vibration the weights are subjected to, and how they might be braced for "G" forces that happen in maneuvering flight.

Epoxy one side of a weight to wood in the engine compartment, and you will soon have a bomb drop. Same goes for those that glue a tire-weight strip of lead to the fuse. side in the tail section. Where it done go? [:-]
Epoxy weights within a scrap-wood box in the frame work and they will probably stay a long time. Lead shot wrapped in thin plastic such as Saran Wrap, stir in a lot of epoxy and then epoxy within structured areas will conform to the structure and hold in very well.
My preferred weight is the flat round fishing weights found where fishing supplies are sold even such as Walmart. They come in different weights and are marked. It is easy to drill a hole in the center and bolt them in. Epoxy can be used for back-up.
Do not depend on the wire line-attachment loop to hold them in place. THAT HAS been tested. []

Good luck.