before the update I defeated the 2nd boss on only day 14 and I immediately got a swamp invasion. since the update multiple days passed and nothing but a few brutes here and there and the usual greylings (I'm in a meadow close to black forest and only somewhat close to a swamp) I really can't say for sure but it might depend on the day you are on I'm past day 30 now and its pretty chill still.