My understanding of it was that the TVA, existing outside of traditional space-time as it does, would have time pass regularly within itself and be completely removed from the time of regular space. So he could have been there for 3 years from his perspective but those 3 years are entirely unrelated to the time happening in the regular universe so it wouldn't even be possible to state how long he's been there relative to the normal universe because there is NO relation to the normal universe. Since he can pop in to the Universe at any point in space-time there's no real way of comparing the two.

I would say it's like asking someone how long it took them to learn Photoshop, like how do you answer that question? It's too vague! Are you asking how long it took to learn a set of core skills within Photoshop? Or are you asking how long it took to master it (which any honest user might say isn't really a thing because there's always some feature that you just never use because it never comes up in your line of work)? And even then, how do you judge how long it took? Do you measure it in the number of hours you spent actually learning the software? Do you measure it in days/weeks/months/years of total use of the product? Does every second of use count towards the "how long it took to learn this" metric? Or is it just the seconds of use that were spent trying out and perfecting the use of different tools that you hadn't touched before?