We try 
It’s 6 months as this is the term stated by HMRC in relation to VAT:
You must wait at least six months from the later of when payment was due and payable or the date of supply. The due date for payment may be determined by your normal credit terms, or by any longer period for payment which you agree with your customers.
(Source: HMRC VAT Notice 70018)
At present, the system handles this all for you. I think if we introduced a customisable range, it may cause confusion for those who aren’t familiar with this notice. We try not to complicate things.
There isn’t a way of doing this unfortunately, although the automated system would send the notices for you as and when needed if you prefer. There’s more on this here. In addition to the guide, it is also possible to disable them for individual clients. I’ll arrange for the guide to be updated with this information shortly.