It's never been definitively established how long he holds a charge, however a pattern has emerged from my forty years of (mostly silver and bronze age) comics reading:
1. It would take a long time for his powers to fade if he left a yellow sun system unless he comes into contact with a red sun system. Therefore, he can fly between solar systems as long as they are yellow sun systems
2. A red sun, however, neutralizes the yellow radiation almost instantly and he loses his powers almost immediately.
3. Leaving a red sun system, assuming he had a yellow sun charge recently prior to that, he regains his powers almost immediately.
1. It would take a long time for his powers to fade if he left a yellow sun system unless he comes into contact with a red sun system. Therefore, he can fly between solar systems as long as they are yellow sun systems
2. A red sun, however, neutralizes the yellow radiation almost instantly and he loses his powers almost immediately.
3. Leaving a red sun system, assuming he had a yellow sun charge recently prior to that, he regains his powers almost immediately.