Free admission to Prado Museum

I assume that you cannot book. There will be no ticketing line and doors are just open to everyone. It's definitely worth a visit.

'Does anyone know if you need to (or can) book an admission at those times in advance.' - You queue like everyone else.

'Has anyone ever gone in' - Yes, several times over a score of years, I have never paid.

'and how crowded is it and how long is the line up to get in?' - It is impossible to forecast how many from preschool or wherever might be interested whichever night. What is your definition of crowded? :-)

If this is your only shot to see it, it shouldn't matter how crowded it may or may not be. You can determine that when you get there, and since it's free you haven't lost anything if you decide it's too much. I would use the Prado's website ahead of time, though, if there are particular pieces you want to see. With only a brief window of time, you will need to go as straight as you can to those, then work your way back till you need to leave.

This is a wise use of your time and well worth the effort. The museum is huge so you have opportunities to go where the crowd ain't. The quality of artistic offerings is so high and vast that you are enriched for whatever you view. Enjoy!

We went at 6pm on a day in April, and it wasn't bad at all.

The Prado is wonderful. We will probably go to Madrid again just to see it again. With short time availability I would focus on the Spanish collection e.g. Goya, Velasquez etc. It is really great to be able to see so many paintings by these artists. When museums are free there is no 'booking' -- even those with tickets or a pass would have to line up like anyone else.

I was there in February during the day and it was nearly empty. At 6, it started getting crowded and I left, since I was exhausted anyway. Since it's the only time you have, you should certainly do it. There's a no-bag, no-photo policy. If you have large bags, you'll have to check them and there aren't many lockers availabe, so don't take with you what you don't need. The Prado is huge so you need to plan in advance. Since it is likely to be busy, it will take a little longer going through the galleries. Figure out which rooms or artists, or periods, etc., that you most want to see and map them out. I would head first to those farthest from the entrance, since most people amble along from the first rooms onward, so those remotest ones will be the least busy.