Hi Hashmit,
Please note that there is no direct relationship between priority and the earliest start and latest finish dates of the maintenance plan. In App 10 version, when a calendar based maintenance plan is created (irrespective of the priority), it will get planned with 1 week duration. However if you need, you can change the dates manually.
However in IFS Cloud version (22R1, 22R2 versions onward) there is an option “Expand backward” and “Expand forward” which allows to amend the planned period for all the maintenance plan lines at once (within the PM Action).
Hope this helps.
Best Regards,
Thilini