Bigsby has ya on the right track. 2-4 weeks (or so) of PMS, then they'll start filling-in. (PMS - Pre Motherhood Stretch) Small growth spurts from there, but most of the action is early.
If you're running out of room, likely a good idea to start the training process now. Bad idea to wait till you run out of space.

Use a little care when bending. The branches are usually quite pliable, but use care and don't snap the branch(es). Also, don't tie the twist-tie too tightly around the branches. If too tight, it might cut into the stem as the plant continues to grow. Don't tie any branches from one plant to another plant. Don't untie it, or the plant will spring-back into vertical growth and the bend(s) will be minimal, but check the ties every week to make sure they aren't cutting into the branches.