If I recall my troops back after sending reinforcements, will the base be defenseless?

Bases are defended by their own troops, if they are at home and set to fight against attackers, and by any other reinforcements sent to help. If you recall your troops then they may be other reinforcements and the bases own troops. If there aren't ant of these then yes it will be defenceless.

There is not set time that you need to leave reinforcements at a base. You recall them when you need them or when you believe the danger to the base is over,