There are many items left undefined; they are left up to the GM
There are several items in the Player's Handbook which are left undefined, such as the Mess Kit, Grappling Hook, Hammer, and Chain; and others from various modules such as garlic, and salt from Curse of Strahd. Some items we learn their weights or prices, but many we don't. Amongst only the various Pack's and Kits in the Player's Handbook the following are mentioned, but never defined in any way: String, Bell, Censer, Alms Box, Bag of Sand.
Blocks of incense are a similar item, they are not specifically defined in the rules. Thus, how they work would be left up to a GM.
Unfortunately, we also cannot determine the price of the incense by summing up the price of all the items in the Priest's Pack and comparing it to the Pack's own price (19gp). Though this does work for other Packs (The Diplomat's Pack costs 39gp and the sum of everything in it is 39.24gp).
What we can say is that an Alms Box, 2 Blocks of Incense, 1 Censer, and Vestments collectively cost around 14.7gp. None of these items have a price though so we are quite stuck. Fine Clothes (probably close to Vestments) already cost 15gp so we can't quite use those either unless we assumed everything else was free.
Incense is used in some other places:
The Acolyte background gets 5 sticks of incense.
There are also spells which use incense as a material component including (but not limited to): commune, conjure elemental, control water, divination, find familiar, and forbiddance.
The Dungeon Master's Guide also mentions incense when discussing religious observances, when listing items a shop may have, and lists an Incense Burner as a religious article.
Tomb of Annihilation also has an item called "Insect Repellent" which comes in two forms, a greasy salve and a block of incense. This insect repellent incense block costs 1sp, has no listed weight, and has a specific effect of repelling insects near it while lit. This is certainly not the same incense found within the Priest's Pack, but it is an example of a specific type of incense somewhere else in the rules.
Unfortunately, in none of these places is incense further defined, nor given a price/volume or price/weight.