It ticks once, so, just refresh it right before it ends as normal with pandemic. Hand of Doom doesn't affect the way it works. It will still tick at the end of the duration.
E: FWiW, it goes up to 24s. So, you can refresh it any time sub 4s and be fine.
The thing is normally you use HoG when you are at 4 shards not when doom is sub 4 sec, so after like 2 casts I kind of lose track of the tick timer, and it helps a lot to know so, for example, I can cast HoG at 3 shards knowing doom will tick before my HoG cast finishs, or when I'm at 4 shards but I can fit one more bolt before call DS comes off CD but I will shard cap if doom tick at this time (If doom would tick I would life tap or cast HoG instead). thx a lot anyway mate