I was told anytime. I didn't need to wait til she had any injections. They chlorine kills any germs anyway.
It's just the younger they are the colder they'll get quicker. So you just need to keep an eye on her temp.
When i took Angel it was a weekday, and the pool was freezing. I was told if i'd gone in the morning there was a class on and they turn the temp up for the babies. It was so damn cold even for me.
So maybe a class would be better, or if you know of a warm pool.