In reference to the classical empires of Europe: decades longer, had WW2 been avoided (the colonies were probably destined for freedom anyway, but the war broke control, and the banks, for most of Europe), and quite possibly the end of this century had WW1 been averted (where the potential capital/political will lost in both wars would have enable a much stronger, longer grip).
If Imperialism went out by 2000 in a world without WW1 and WW2, I'd be willing to bet that it would be a result of low-scale Imperial Warfare, with empires funding insurgents in each other, rather than popular independence/decolonization movement we saw more of in OTL.