From the DMG on page 133 (very bottom of the final paragraph):
Over the course of a typical campaign a party finds treasure hoards amounting to seven rolls on the Challenge 0 - 4 table, eighteen rolls on the Challenge 5 - 10 table, twelve rolls on the Challenge 11 - 16 table, and eight rolls on the challenge 17+ table.
In an earlier paragraph it explains that each treasure hoard should have at least 2 rolls on the appropriate table and that if the hoard seems too small you can roll keep rolling on the table. This makes it so that you as the DM are free to distribute treasure at a pace that makes sense for your campaign. In general the DMG tells us that there should be a treasure hoard when it makes sense in the fiction.
So for example the 7 rolls on the Challenge 0 - 4 table can be divided into two hoards: one hoard found that you populated by rolling twice on the table and another hoard consisting of 5 rolls worth of treasure. Or 2 hoards of 3 and 4 rolls. Or 3 hoards with 2, 3 and another 2 rolls.
It should also be noted that the different challenge tables don't necessarily correlate to the tiers of play. If the party hits level 5 and you haven't hit all seven rolls on the 0 - 4 table don't sweat it. Even at high level the party can still encounter lower CR monsters that would have hoards from the 0 - 4 table.