Is there any limit on how long this prorogation may last?
The limits can only be decided by a court, because the UK does not have written constitution setting them out.
The PM advises the queen to prorogue Parliament. There is an unwritten rule that the advice should be given in good faith, for the smooth running of parliament rather than any political end.
A court would have to decide if any particular advice to prorogue was given as such.
There is currently a case before the Scottish courts on this very issue regarding Boris' proroguing.
From a legal standpoint, I've been unable to find any evidence of a limit on the length of prorogation, other than that prorogation cannot continue beyond the date when the current Parliament would end prior to a general election.
Hence, following on from what "user" points out in their answer, so long as no court rules that the advice to prorogue was unlawful (which, at the time of writing, has never happened), a Prime Minister could use this power to prevent Parliament sitting again until after an election.
From a practical standpoint, there are a number of problems that would occur if parliament was not able to sit for a long period. These are discussed in answers to this related question.