I'm going through a long complicated list of jobs and activities, some are not very recent. Is it wise to write "est." for estimated next to every single entry if hours and dates are estimated?
No, that's not necessary. They will not expect that you know you did exactly 2173.54 hours in an activity ten years ago.
Make a broad calculation using your knowledge of how many hours per week you worked x weeks and put that, and use your best judgment on the date range if you don't remember exactly. That's how I managed most of my older activities.