Whoever wants to get in the fight can do so right now.
The Ukrainian government looks to have created its own foreign legion, so you'll even technically be fighting under the Ukrainian flag as a uniformed soldier.
Enlistment period is indefinite: until the "state of emergency" ends.
Update based on new information: Russia doesn't acknowledge the Ukrainian Foreign Legion (they keep rebranding, not sure what they'll end up sticking with), so if you get captured, the Russians will treat you like a criminal, and not an EPW.
The Ukrainian government looks to have created its own foreign legion, so you'll even technically be fighting under the Ukrainian flag as a uniformed soldier.
Enlistment period is indefinite: until the "state of emergency" ends.