That's the information you would ascertain from your experiment. If we supplied the information, then it wouldn't be considered an experiment. Right? Unless you're wanting to use an existing theory. A good experiment usually tests an hypothesis which basically adds support to the original hypothesis. Therefore, I guess you are correct in asking this question. Did I not just talk in a riddle. I'm really good at that.

One suggestion I have is that a gallon size wouldn't be considered shrub size.

Aren't you glad you asked? Oh, wait. I have an idea. Go to your nearest Home Depot and pose this question to an employee in their garden department.

It depends what you are growing. If you take a cutting of an azalea or rhodo in september, I would put it into a one gallon, the next spring, and then by the following june or july, it would be a nice sized one gallon. That's about 22 months?
Some things grown quicker... hydrangea, azara,choisya, cedars...and some things are slower.]Variables such as fertilizer and zone you are growing in will affect the rate of growth.
Good luck on your experiment. I hope this info helps.
Maplea