How long do the royalties typically last?
That is not a simple answer. It will depend upon the type of well, vertical or horizontal, reservoir, completion type, depth, pressure, oil, gas, etc. Cannot answer you unless you give a location and even then it would be an estimate.
Washington county PA w/ eqt. If royalty checks go down by 25% each month (last 5 months) will they stop for good at zero?
If you are talking about a horizontal well, they usually have a steep decline for a while and then they change slope to a more reduced decline. Some horizontals can last decades. It is called a “hockey stick” profile. The blade of the stick is the first few years and then the long stick are the remaining years.
Rights to mineral interests are forever, Royalties occur when the minerals are extracted and sold. if production stops, so do the royalties. Leases can bind mineral interest owners and their descendants for decades or longer.
Our experience in the Marcellus is that the royalties are lasting far longer than they first told us, partly because they refrack the better producing wells after a few years.