Is it a monthly subscription? |
No, it's permanent.
or once I purchase it I can use it for as long as I want? |
Yes... so long as you are on the same OS and hardware. After a few years, those do change and the compatibility is not guaranteed. New versions of Windows and Macs as well as new hardware changes may create problems. For instance, I have been able to re-install versions 5,6,10 and up to 15 versions of Elements, but other users have problems requiring special adjustment in the OS. Or new high resolution displays were not known for Elements version before 13.
Also note the system requirements, for instance Elements 15 requires a 64-bits OS and hardware.
Tech specs & system requirements | Adobe Photoshop Elements 15 for Mac, Windows, PC
Tip:
Use the trial download to test with your hardware.
If you buy after, you don't have to redownload, you just enter the product key given for your purchase.