For 300 tasks you'd need to ask thousands of questions.  No one is going to do that, and even if they did the results wouldn't be very useful.

For situations like this I'd suggest you stick with the requesters simply assigning an importance factor to each of their requests.  If you have competing users, the software can also have a hidden user-importance factor that weights each requester's own importance.

But if users can already assign weights to their requests, why not simply let them change those weights whenever they want.  Give them tools that they can use to meet their needs, and there won't be a need for scheduled task reviews.

And don't let your system (or the users) confuse "importance" and "urgency".  For instance buying a birthday gift for your spouse might be very important, but if their birthday isn't for another 6 months it isn't urgent.  Similarly "come here and look what this guy across the street is doing" is very urgent (he'll stop doing it soon and you'll miss it), but it really isn't important.