Realistically, you'd need probably 60-70% of hash rate to pull off a sustained 51% attack.
144 blocks/day...having 51% means you would on average mine 3 more blocks/day vs the other chain. Luck comes into play here too...the minority HR chain might have slightly less than 50% of HR, but over the next hour maybe they find 5 blocks and the majority HR chain finds 1.