These post-communist days, good. After 1989, Poland entered the capitalist world with a vengeance and has spent the last 9 years making up for lost time. You can spend a lot in the boutiques, or a little on the sidewalk vendor or in the open-air market. You can get some bargains if you seek them, for Poles love to haggle (so bone up on your Polish numbers and your Polish sizes; they'll come in handy). If you're looking for items particularly Polish, be on the watch for wooden carvings, amber, lacework, linen embroidery, and posters. Polish posters are renowned, and can be gotten quite reasonably in the off-tourist sites.
If you go into a self-service store ('samo-obsluga'), remember two things: get a basket and watch your place in line. Although the stores have changed, some of the communist habits remain. Baskets, which regulate the number of people in a store, can usually be found somewhere near the entrance. If you don't see any, wait until a departing customer empties one. Note, you will be chastised if you are without one. Even if you're a tourist and a foreigner and don't understand Polish, they won't let you get away with it.
Once you've found what you want, get in line. Don't be surprised if the person behind you stands rather close. Back in the day, standing in line was the national past-time. A small gap between you and the person next to you might be exploited by someone more on the ball, increasing the already lengthy time in line.
And once you've got your goods, you might have to ask for a bag ('torebka'). Sometimes they remember to give one, sometimes they don't. To avoid this uncertainty, you can simply carry one with you.
Yes. Look for 'nonstop', or '24 godz.' or 'cala doba' on a shop window. There aren't many, but they do exist.
Is anything open 24 hours?
Otherwise, expect to go grocery shopping anywhere from 6.00 to mid-afternoon in a rural area (later in the major cities). Non-food stores open around 11.00 and close around 18.00 (plus/minus an hour). Most hours are shortened on Saturdays, and non-existent on Sundays, so keep that in mind if you're looking for a special gift.
Likewise, museums are often closed one day of the week, and tend to bar new visitors at least one half hour before actual closing time (some even stretch it to an hour before).
When can't I shop?
On holidays, mostly. Unlike money-mad America, stores and (just about everything else) do close on holidays. Check below for the critical dates.
New Year's Day
April 13-14, 1998
April 5-6, 1999
June 11, 1998
June 3, 1999
All Saints Day