Greetings Bhattacharya,
You will need to create a Multiple Counter Plan. Therein you may specify the Cycles as 250 run hours OR 6 calendar months. You will also need to create - and specify on the same plan - a Measuring Point for Characteristic "RUN_HOURS".
So, the sequence is:
CT04 - find or create a Characteristic with the type NUM and unit H
IK01 - create a Measuring Point with this Characteristic for your technical object (DG set). Specify an Annual Estimate (for continously-ran Equipments it's abouit 8000 hours, but for your assets it may be less)
IK11 - create a Measurement Document for this Plan
IP43 - create a Multiple Cycle Plan with these parameters - note the cycles and the -OR- link.
Assign a Task List to the Maintenance Item and you're good to go.
I hope this answers your question, but follow up if your requirement is more complex 