Difficult question:
To answer your question, one will have to know how many DLR applications have been made within a given time period and the time taken to decide each of those applications. It is not matematically possible to calculate average time without these two vital pieces of information! I don't think anyone on the forum would have this information collated anywhere. You can search for timelines posted on the forum or make an FOI request to get this information from HO, and then calculate the average.
All anyone here can do is guess!
My "guess" is that it can take anywhere between 3 months to 18 months for a DLR application to be decided. Do
search for other guesses / timelines already posted.