Well, almost every week. They fairly regularly skip weeks. And they tend to switch the start times quite a bit, particularly with the Wednesday Night show. Addtionally, the don't run all year long. FNF runs roughly January-September, WNF April-Ausgust. The last WNF was, I believe, this week.
Every week during it's run, which is generally spring and summer. Both WNF and FNF are going on hiatus soon as ESPN2 picks up it's college football coverage.
That's why I am always reluctant to criticize ESPN, even when their matchups aren't the best. They do make a commitment to the sport, and the opportunities to see the boxing on basic cable would be practically non-existent if it weren't for ESPN. The Versus network shows a card on basic cable about once a month, and Fox Sports Net has a card every once in a great while, but it's ESPN that carries the load.
I think that ESPN's boxing shows are mainly an excuse to cover the sport. Most of the time the fights are pretty lame (there are always exceptions), with lots of unkown fighters and others with dozens of losses in their records. Cards seem to be a little better if they're held outside the USA. They show clips from the weekends fights, and on fridays a boxing personality is invited to the studio. Not the greatest boxing show ever, but better than nothing anyway. The problem seems to be that ESPN cant find a good sponsor for the show. This season the sponsor was a ridiculous hair dyer for men, and I'm not joking. That means low budget, so mostly second rate fighters are shown.
This past Saturday was the last WNF's show and September 7th is going to be the last FNF's show of the year and will be back in January.