There isn't a certain time period that is set for any tanks as all set ups are different, with differing numbers of seahorses in any given tank, and, owners that vary in their husbandry protocol. Also, types of mechanical filtration can affect frequency of various tasks as IMO, mechanical filters of any kind need to be cleaned at worst every 5 days, more if the tank is heavily loaded, but preferably about every other day so that less decomposition of the trapped particles is occurring.
Seahorses produce "very dirty water" in that for the most part, most don't eat all the pieces of mysis added to the tank, preferring to be selective and only eating pieces that in their eyes at least, are perfect specimens.
This means pieces getting trapped out of sight at the base of rocks or between rocks, and caught in decor/hitching/macro.
As these pieces degrade they become bedding and fuel for nasty bacteria as well as for algae, and bacterial problems are probably the number one cause of seahorses problems/deaths.
In addition, when they snick up the food pieces, they masticate it before it enters the digestive tract, and pass minute particulate matter out through the gills, again degrading the water quality and providing more fuel. (This often looks like smoke coming from the gill area upon snicking)
The more water volume you have over and above the minimum recommended levels, the more forgiving the situation will be, but in the end, insufficient husbandry will lead to problems, but just a little later on.
Unfortunately there are no tests a hobbyist can do to check for the conditions so it's a case of starting off doing a lot more than what you would for a reef tank, especially vacuuming up any trapped detritus/uneaten food.
With my bare bottom 40g seahorse tanks I prefer to do 10g change after a week, and 15g after the second week, 20g after the third week, and 30-35g after the fourth week. Not everyone feels the need for this level but it works for me.
When I was doing less, I frequently had problems.
