How long can pokemon safely stay in mega form?

That depends on how new to mega evolution they are, anonymous. Those extremely new to mega evolution may only last a few minutes before losing control of themselves or before their bodies simply automatically revert to their normal forms. However, over time and through training, a pokémon may achieve longer and longer periods of mega evolution until they could, in theory, remain in that form for as long as they’d like. It’s simply that mega evolution can be an extremely draining state to maintain, but like any form of exercise, it gets easier (both to achieve and endure) the more one does it.