When to attack an enemy planet depends on how fast you can strike!
A great time to send forces is when your enemy moves before you, and your Squards are two turns away and you have enough energy to grab initiative, or can trick another player into taking initiative so that you go before your target. Then, moving in a turn after your target, you send your squad(s) toward them, set initiative either by purchase, tricky or blind luck and gain the double move to be able to attack their planet before it can reinforce.