short periods of 20-30 mins are fine and another thing is to make sure bubs can put feet flat on the floor. I was concerned about hip/spine issues and asked a physio what she suggested. I actually tested DD's in the doorway by hanging off the bracket myself prior to putting her in it. There is also a pigstail hook mount that can be put directly into a beam if you're worried doorway isn't strong enough.

HTH!