Login or register (free and only takes a few minutes) to participate in this question.
You will also have access to many other tools and opportunities designed for those who have language-related jobs (or are passionate about them). Participation is free and the site has a strict confidentiality policy.
Explanation: Unfortunately, Ivona, you do not give any context of the expression.
Two main meanings spring to my mind:
1. DŁUGIE CZASY NAŚWIETLANIA/EKSPOZYCJI
In photography: When the camera shutter is open for a long time (usually longer than 1 second). The situation occurs when there is little light available, so the light sensitive film must be exposed to light for longer period.
2. DŁUGOTRWAŁE WYSTAWIENIE NA DZIAŁANIE (jakiegoś czynnika)
LONG EXPOSURE OF METAL OBJECTS TO WEATHER CONDITIONS.
the expression needn't have a negative meaning. Long exposures to some agents may have beneficial results.
Hope that this helps you out, Ivona.
English-Polish Dictionary of Science and Technology own experience