Is it just mileage sensitive ?

On my CDI Smart car it took into account both mileage and type of driving. If you did a lot of highway miles at speed it would go over 5k miles before requiring a change.

The reason I ask is that I just went over 5k miles and the indicator was flashing on start up but did not stay on. My understanding was that when you hit the full mileage it stayed lit ?

it is just a reminder based on whatever mileage you set it for. it does not know whether or not it needs maintenance.

IIRC, the MAINT REQ'D light starts to flash when it's within 500 miles of the OCI, then it stays on steady when it's within 100 miles of the OCI. CMIIW.