I have run many reactions of DMF at reflux overnight, and there has never been a sign of any real amount of decomp. in them at the BP for 16+ hours. But as stated, there is almost always a smell of DiMeNH2 in it, so there must be some small equilibrium to the decomp. at RT. But it is fine for high temps. If DMF or DMA is not ideal, NMP is the best thing, as it is nearly impossible to decomp, even with conc H2SO4 in it.