how long before adding new hens?

Yesterday, one of our 3 girls became ill and was dead by the end of the day. We've found its best to add 2 new hens to 2 current hens, rather than to just one (they're pets, we don't want lots) so we were going to get 2 new girls, but how long after Pepper's death should we be waiting? We don't want them to get ill

It's good practise to quarantine the new ones as you don't want to risk them giving your original hens any infections  ( and vice versa as you've pointed out)