3-5 years, depending on growing conditions. Unless you are located in a cold winter zone where the plant may experiences significant winter die back, in which case it could be longer.

In 3-4 years mine have hit 3x3' (whether or not they get killed all the way back).

If they get killed back then i have to cut them down right?

But if they dont get winter kill, will the bare branches leaf out?

Is it perennial like a hardy hibiscus that comes back every year?

If the branches are alive, they should leaf out no later than mid to late May. (earlier in the South). If you see no leaves by mid-to-late May then you can assume the stem is dead.

Yes, the hibiscus should if it in zone. I have a hardy hibiscus (hardy in my zone 8) that has come back the last ten years ++.

You are zone 7 and based on my experience please examine your "south facing" site carefully. I have two T & S (zone 6a) and three other macs in south-south west location. Regardless of how much they are watered they wilt in the afternoon. Some people are able to ignore this, but I think it is really unattractive. It is also a pia to water them as often as necessary in full sun. I'll be moving all my macs to part shade, even at the risk of reducing flowering, I know it is the strong afternoon sun that's the culprit. I did not know much about hydrangeas when I planted them.

My T & S have been in the ground about 3 years. We had a terrible winter. I'm embarrassed that I can't remember as far back as April as to whether they were killed to the ground or not. However, even if the old (last years or older) stems are killed, the new growth will have flowers. It may just be later blooming. In a matter of weeks T & S puts on feet of growth. Mine are very vigorous. And they keep pushing out new flowers, unlike -it seems to me- some other macrophyllas that have one big substantial bloom. To state the very obvious, if the roots are killed, it is dead and won't do anything in the spring. But is very root hardy, too.