This isn't timed, but the effect is the same. You set a number of tabs you want active at once. You mark some tabs as "can't be made inactive". You decide whether those count. And then you browse.

Beginning max is 25; I set mine to 10 and marked a few tabs as un-unloadable. The effect for me is that half of my pinned tabs are always loaded and the other half are unloaded, which was what I needed.

Side note: If you use Sidebery, switch on the native context menu in the settings and add your keep-loaded tabs, then turn it back off. I mention this because it came up for me and I'd forgotten about that extension.

All in all, good job, thanks!