There are no hard and fast rules. You need to establish some sort of baseline of acceptable accuracy and consider barrel cleaning when performance falls short.
Every gun is different and so is every batch of pellets. In my own Walther I had a batch of Exacts in 2007 that were very good but required barrel cleaning every 500 or so shots as accuracy fell away.
Since then the good Exacts I've had have not given me cause to clean the barrel. I did clean it about a month before I went to Italy, after about 6 shots the accuracy was identical to before cleaning.
I don't size or weigh my pellets, I use them straight from the tin, I just measure how long they are. I've never had Exacts shorter than 6.18 mm not group and I've never had any longer than 6.22 mm that do. I just measure a couple from each batch as length consistency in each batch is superb. In my particular gun short Exacts will hit a 13 mm bell target indoors at 50 yards, without exception - assuming I point the thing at the target.
For what it's worth, I clean mine with surgical spirit on a bit of cloth and drag it through using nylon fishing line. You might need to use a drinking straw to feed the line through your EV2's muzzle thing.
My FWB P70 never needed cleaning, the barrel was sparking clean when I did pull it through, my black Ripley was the same.
I might actually come to the club tomorrow, let me have a go with it, I'll see if I can get it to group; if the gun really is throwing a few about there are any number of things that might be up, but cleaning the barrel is an easy place to start.
My own personal experience of my gun is not here as a template for what yours might or might not need, it's just an illustration of the way one gun is; you need to find the treatment that works for yours. The dominator next off the production line might need cleaning every 300 shots....
Roger lent me two aparently identical MK2 Protargets a couple of weeks ago with very close serial numbers. In one gun the pellets sort of fell into the barrel, in the other I got a sore thumb shoving them in - there is no way these two 'identical' guns are going to behave the same way.
I think you should maybe clean it and come to the indoor 50 yard range and really see what's happening - establish that baseline.