As you will see on a number of threads here, the SUP does not apply to BB Plus.
The reasoning is that Pn themselves manage the connection quite firmly so you shouldn't be able to use it to too much.
However, you are still subject to the Acceptable Use Policy, which means that if you use your connection so much that Pn believe you are in danger of impacting other users, then you will get a warning email.
So there is no fixed limit, but Plusnet can, at their discretion, decide you are using it too much and tell you so. There are a couple of reported instances of people on BB Plus receiving emails telling them to ease off.
It's not as clear or, in my mind, as sensible as simply giving a figure like the SUP one, but that's the way Pn choose to do it.