ISC DHCP server config option adaptive-lease-time-threshold
The adaptive-lease-time-threshold statement
When the number of allocated leases within a pool rises above the percentage given in this statement, the DHCP server decreases the lease length for new clients within this pool to min-lease-time sec- onds. Clients renewing an already valid (long) leases get at least the remaining time from the current lease. Since the leases expire faster, the server may either recover more quickly or avoid pool exhaustion entirely. Once the number of allocated leases drop below the threshold, the server reverts back to normal lease times. Valid percentages are between 1 and 99.
A good idea for a lease allocation hook library.