How long before mealworms turn into beetles?

Mealworm is a common live food used in feeding pet birds and reptiles. They are quite available from many places. However they have a limited usefulness before they turn into their adult form - mealworm beetle.

For those people who have used mealworm quite often, how long before your mealworms turn into beetles?
What can you do to slow their growth without reduce their nutritious value?
What are some warning signs that the worms will start to turn?
What do you do with the mealworm beetles?

By the way in your opinion, are canned mealworms as good as live mealworms? They claim to be better than freeze dried which I know it's not as good as live worms.