It's impossible to know, unfortunately, as it's impacted by the behaviour of other applicants and the other universities to which they applied.
The most likely reason that you're on hold is they they've made as many offers as they're comfortable making right now. They can't make too many offers, as they have limited places on the course. So they're waiting to find out if the applicants to whom they've made offer choose Bristol as their firm or insurance choice. If they don't, then they have the capacity to make offers to some those currently on hold. However, the students to whom Bristol have made offers can't make their firm/insurance decision until they've had all their decisions back from the other universities to which they've applied. And those universities may be holding off making offers until they know what their current offer-holders are going to do. The whole system can seemingly grind to a halt as a result.
Hence why it's impossible to know when Bristol will be in a position to make any more offers.