Not owning a 3D printer, my knowledge from first hand experience is nil, but I have done some research with a view to purchasing a printer at some stage. Part of that research involved purchasing "3D printing for model engineers", written by our own Neil Wyatt. In it he makes a number of observations regarding worn nozzles, notably those made from brass which should be considered a consumable. He suggests that they should be changed every two spools of filament. More often, if the more abrasive filaments, such as carbon fibre impregnated nylon ones, are used. More durable nozzles for this type of filament can be had with a ruby insert, for example, though they will be commensurately more expensive.
Symptoms include blobby prints and over-extrusion, apparently.
I can recommend Neil's book for the beginner – it contains a lot of useful information and guidance as well as set up advice and trouble shooting. And, no, I'm not being paid to say that!
John