Thats a very open ended question, i would say generally about 1 WPG, but 1 WPG over a ten gallon tank is a lot different than 1 WPG over a 100 gallon tank, very little may survive in the 10 gallon whereas lots may survive in the 100 gallon.
But again it depends on the tank in question, some people seem to have luck with very low levels of light and others with the same tank could have no luck at all.
About 1.5 WPG is a nice level of light for a low light tank IMO, but plants that may survive at lower levels i would guess would be Java moss, Java fern and some Crypts might do well also.