when can i replace nhs dentures?!

The NHS will only replace when they are no longer fitting correctly or cannot be repaired and there will be a time scale attached to it as well.

What you are going to find is that very few NHS dentists will now make dentures for patients under the NHS. They are allowed to mix and match NHS with private under the current contracts and they simply cannot afford to make them for patients under the amount the NHS pay. The time and effort and materials needed, repeat appointments and laboratory fees mean they work at a loss or there is a compromise on laboratory and denture finish.

I think you need to ask your dentist and he will decide if they need replacing under the NHS. Apart from the dental aspects it depends on several things, such as what benefits you are in receipt of (if any), over 60/65 and in receipt of pension credit, in receipt of housing benefit etc. If you are entitled to change them under the NHS then you just need to sign a dental form having shown them the relevant proof of status with regards to benefits etc.