see - How often should a node do an OTAA Join, and is OTAA better than ABP?
best practices - https://www.thethingsnetwork.org/docs/devices/bestpractices.html
security - https://www.youtube.com/watch?v=6IDaUxhEgaI
activation is normally a matter of seconds and probably access neccessary to set the keys you received during registration of the device (depends on bluetooth/cable/wifi)
once the device joined the network otaa it can for example go to sleep for a month, and when waking up
transmits again without re joining, the session don’t expire.
for security reason it would possibly be better to rejoin otaa after xx transmissions