Nobody knows exactly. They sometimes dig even when not mating. They like to spawn in clay flower pots very much. If you put one in, make sure its clean.
Watch them closely and you will know what type of things they do when they get ready to lay the eggs and how long it takes for them to hatch. It really depends on the fish and its hard to say without some pictures.
It is very likely that they are spawning though. It doesn't take much to get them in the mood.