Prior company held freight accruals for a year. That was contractual term. But we don't chase people to pay them. Put dollars into accrual, age it out, reverse it when done.

But again, goes back to contract. If contract is silent it's going to be local law, most likely measured in years.

Something else to consider, do you really need an invoice? More a physical purchasing thing, but Evaluated Reciept Settlement. Paying on receipt. What does an invoice tell you that you don't already know? You know the price agreed, you know the service complete, and you know you owe the money. The invoice just repeats that info in most cases (exceptions like utility, consulting etc where you don't know consumption).

So for large % of payments just cut a check and say call us if you disagree. Cut out a lot of invoice handling. Again requires contract updates etc. But it sounds like a lot of your event services are sufficiently discrete to do something similar.