Any suggestions would be much apprecaited
I am afraid that there cannot be a single answer to this as it will always depend on the nature of the PMI. An instruction to make scaffolding safe where it is erected on a busy highway probably needs to be reacted to within hours (or possibly minutes) whereas a PMI to change the planting mix alongside a road when you are only marking out the alignment could be reacted to in weeks.
The long stop date, as a good rule of thumb, is going to be the period for reply in the contract as it would be very difficult to justify saying nothing for longer than that. However, you should be aiming for as soon as is reasonable in all of the circumstances of the PMI