hi, AS :)
the jam is already ultra-casual while "Make a game that lasts only 20 seconds!" pretty much says it all. and, to re-enforce the notion: the 1st FAQ.
so, the game cannot exceed 20 Seconds, but you could make a 10 Second Game with 2x 5 Second Bonuses inside :)
personally, i find it a fun, unique challenge that has kept me coming back for years, and i love seeing the FUN that hundreds of others have fit into what might be considered a generous amount time (i've participated in jams with less) while most participants find 20 Seconds to be "just right".