thank you all for your feedback.
according to
http://bitcoinwisdom.com/litecoin/difficulty the last two difficulty increases since the bitcoin boom end of november have been 13-14% per 2016 blocks. projecting that to the future would mean no GPU will ever pay off. see
http://bitcoinwisdom.com/litecoin/calculator using 13.5% increase.
however, i expect that the difficulty increase will flatten once the current hype wears off. but since i'm rather new to mining, i lack the experience if, when and how that will happen. any experienced miner around here who can share his experiences with earlier booms?
@jongameson: mining altcoins can help with productivity. right now i'm mining trash coins with low difficulty and try to convert them in BTC and LTC when prices are good. while difficulty is no problem with them, i have to consider exchange rates, which are impossible to predict. no use for mining 1,000,000 DyingCrapCoins fast if the exchange rate drops even faster.