My boys are 2.5 and 4.5. My 4.5 year old can be in the tub solo and while I do check in on him and am on the same floor as he is, I don't worry. My 2.5 year old can be in the tub solo while I say get a towel in the hall way for 20 seconds (closet is RIGHT next to the bathroom) or yell downstairs to DH to bring me the comb etc. I will leave them both in the tub solo while I get their PJs (again on the same floor). In general I am in the bathroom with them. They are not allowed to stand up and if they do they come RIGHT out. DS1 is great about this and DS2 is usually very good. They are not allowed EVER to turn the water on.
The boys were never left alone in the tub until they were >2 not even for a second.
Your turn!
/hillary