Mine seems to switch over pretty quickly. I havent sat there and watched, but generally the next time I pick up my phone I am on my wifi network. Same at my house and at my office.
For example when I woke up this morning and the update finished( i had already postponed it the max of 3 times) the only way I could get wifi on was to cycle wifi off then back on. Otherwise it was on 4g. I see no reason for it to ever be on 4g when I'm at home
My wife knows I'm home before I walk in the door since she gets a message on her computer monitor that my cell phone just logged onto our home network.