Fire does not burn for long periods
Wood can contain a number of variable factors related to species, weight, age and moisture content. It is difficult therefore to be precise with regards to the length of burn time.
If it is thought that the fuel is burning too quickly:
The diameter of the logs are too small
If softwoods are being used, change to hardwoods
Excessive chimney draught. If so, fit a draught stabiliser
Worn out door and glass seals need replacing