At what point does it become pointless to use our GPUs?

If the current n-factor gives me 120/150 KH/s for my 280Xs/290, the next will be be 60/75 and then 30/38. I will still be able to mine the same number of coins with these hash rates? Just trying to figure out how long my 4 280Xs and one 290 have got with this coin/scrypt.