It used to be the tradition that lemmings levels have a time limit. Many levels in the earlier ranks have over 5 minutes to allow for the level to be open-ended and easy, while the later ranks usually only give a lower time (for example, you only get one minute in Mayhem 3 "It's hero time").
However, recent levels like those in Lix and Lemmings Plus Omega have started to use infinite time. namida's recent contest has also had us use either 99 minutes or infinite time depending on what engine we used.
What is your opinion? Do you prefer levels to have a time limit, or infinite time?