How hard is it to get a party sticker from an infinite infinity chest PvZ GW2

@LegibleAbyss7 It really depends on how many shards you can collect.

Your best chance of getting part of a Party Time character variant, usually a 1/3 of the character, is to collect at least 25,000 Time Shards which entails getting up to or past Wave 25.......which takes time and from that point of view is pretty exhausting. 

By the way I nearly forgot to say that you can obtain the shards either by yourself or by teaming up with three others at the most.