For me, it depends.
Its a case of considering how enthusiastic I am about a project, what benefit it would have for me to finish it off, what else I've got on, and how much time (or resource) it would take.
There isn't a fool-proof formula for figuring it out though. A lot of the time for me, its about gut instinct - if its a project I'm excited to finish, I'll carve out time and make it happen. If I'm non-plussed about the project, I won't bother.
I've become very comfortable with just binning things off, especially papers or grants that've been through several rounds of rejections. In the past, I'd have just kept on going indefinitely and those papers/grants would just take up headspace. Now I don't - after a few rejections if I'm struggling to see where they'd neatly fit, I just give up and move on.
This is all contextual though. For me, I'm a social sciences SL at an RG. I'm not seeking promotion. Redundancies aren't yet on the horizon at my institution. Even if they were, I could afford to lose my job. I'm coasting along and enjoying it. So, I make my decisions about long-forgotten work in that context. For example, I'll dedicate two weeks to writing a 'useless' (i.e. non-REFable) but fun book chapter, instead of working on a potentially world-leading but boring journal article. In other words, I'm not being strategic in my decisions.
In terms of actually getting down to it and finishing off long-forgotten work, I have a few strategies:
- Fundamentally just carve out time. I'll block a week or a fortnight or a month in my diary just for one task. That relies on saying "no" to a lot of people and things, and being strict with things like meeting and email schedules but it works for me.
- Hiring a research assistant. This year, I've used some of my personal research fund (that we get each year for conferences etc.) to hire a PhD student to work on data analysis and paper write-up with me. It's been amazing - I simply wouldn't have had time to even look at the data and paper without help from her.
- Smushing things together. Where I have projects or data or bits of writing that could be coalesced, I do so. This gives me more to write about, more data t draw on, and decreased the number of things to finish.
- As I said, also just being comfortable to get so far into a long-forgotten project and just go "Nah, actually I can't be arsed".
I can't really speak to the issue of resource for experiments and the like because that's not really a thing we run up against in social sciences. Is there potential to get a summer student or final year project student (assuming they come with an adequate consumable budget) to finish off some of the lab work needed?