How much time do I have to spend for NSF panel? The field is mathematics/computer science.  If one were to be selected to serve on an NSF panel, how much time/effort, realistically, will one be expected to spend?  Especially for someone with no prior experience in this role.
Here's my typical calculation: It takes me ~4 hours to read a proposal (applied math/computational science, but I assume it's the same in other areas) and write a meaningful review. You'll get 8-10 proposals to review for a panel, so plan on a full week of work before you even travel to Alexandria. Then you'll spend 1.5 to 2 completely full days there. Expect to read more proposals on the evening of the first day.

If you have no prior experience, make sure you have a mentor in your department with whom you can talk about expectations for reviews. I see a lot of reviews, in particular from younger and less experienced reviewers, that are really just 3 or 4 short sentences for each of the 3 categories (intellectual merit, broader impact, summary), but these are really not very useful to either the program manager nor the applicant. Be the person you want your reviewers to be and write long, meaningful reviews. You're doing the right thing to your profession, and your program manager will like you too -- which is probably a thing worth striving for anyways. My reviews are typically 1-2 pages long. That takes time, but I think it's time well spent.