Hi Mohan,
it depends on the Operation type you use in the scheduling parameter.
For Example: You have a multiple counter plan say 1000 hrs or 3 months. In the measurement counter you have maintained the annual estimate as 6000, which is 6000/365 = 16.5 per day. Your current reading is 100.
When you use 'AND' type, maintenance will be due when the measurement reading reaches 1000 also the it should have 3 months interval between the last call.
If my last call is on 01.08.2015, system will estimate when the measurement counter reaches 100 + 1000 = 1100.
1 day = 16.5 hrs
1100 / 16.5 = 60.6 days.
Measurement reading will reach 1000 hrs by 29.09.2015, but your another counter 3 months will reach by 29.10.2015, so the next maintenance due date is on 29.10.2015.
If you use OR, the next due date is on 29.09.2015.
Hope this helps.
I suggest you to test in your system and calculate the dates. This will clear the gaps you have in this scenario.
Regards
Terence