The maximum content duration of the the Premiere Elements Timeline is about 23 hours. I have actually determined this limit after which you cannot add anything more to the Timeline. If you need an exact figure, I will search my records for it.
However, how much of that content that can be exported will expend on your computer resources and the complexity of the Timeline content.
You need to define your limits.
I would suspect that more installed and available and usable RAM in your system should be explored. Does 3 HD represent external or internal hard drives, and what is the capacity of each?
If you are referring to cut and paste between Premiere Elements products, how have you decided to do that?
As I have already done part of the "first reel" I don't want to destroy this if I add the "second reel"
You could make a copy (as backup) of the project, work from the copy to add the "second reel".
Thanks, 23 hours is more than enough.
All the HDs are internal 500gb each with plenty of room. I found using 3 HDs like this much faster when I had PE9
Is there any info as to how much ram I should ideally have for a given amount of video of a certain resolution represented by the timeline?
It is hard to know when PE is using the RAM or the disk for temporary storage and the HDs all have caches too.
How many slots does your computer motherboard have for RAM modules? And, do your computer specifications give a computer manufacturer's maximum amount for supported installed RAM? 16 GB installed RAM might be the next step.
If the Task Manager is not giving you enough information, you might want to explore the free Process Explorer Sysinternals.
Considering the variations in the complexity of the Timeline content, please give a mini test run look at defining your limits.