The time it takes to activate a Garmin InReach will vary... Once you've setup your required inReach Explorer account satellite airtime plan, you can then go outside and try to complete the activation process by "Acquiring a GPS fix". The Garmin InReach manual says to wait at least 20 minutes for the activation process to go through, as stated below (but it can take less than 5 minutes or over 1 hour):
"The activation process could take up to 20 minutes. Activation of your inReach with the Iridium satellite
network is a one-time step that uses a different process than standard inReach functions. Please be
patient while your inReach communicates with the Iridium network for the first time."
They also say, "inReach Explorer needs a clear satellite signal from the sky to complete the setup process. Go outside and stand in an open area away from tall buildings, roof overhangs, and trees."
So to get the activation to go connect the fastest to the Iridium satellite, your best bet is to try to Acquire a GPS fix in a open field on a sunny day. It can take as litte as 5 minutes to activate the Garmin InReach or as long as an 1 hour.
I recently reactivated a Garmin InReach SE device, on a winter cloudy, snowflake flying day, while ski touring through trees and it took roughly 1 hour for the Garmin InReach device to finally activate. It activated when I reached the top of the ridge line I was ski touring up to (1,000 feet above, where I first started the Garmin activation process).
Keep in mind, water in the air, or on Trees will make it more difficult to a acquire a satelitte signal. It will be very difficult if your in a Pine forest (Coniferous trees), since pine trees retain water in their needles. You may have little bit better luck in a leafy Deciduous forest, but it may still take a long time to reach a satellite. Your best bet for acquiring a GPS fix with the Garmin InReach is to be in an open area, with no Trees or Buildings around.