When you get that feeling that what you're doing is "like watching cable, only with fewer hair replacement infomercials"[1]. It's noticing the particular kind of boredom you get when you're slipping into a not very interested consumerist attitude.

I have generally treated this as a signal to stop bothering with whatever it is. Could do with strategies for finding more interesting new things to do.

(I find myself uninterested in trying to get more interested in the not-sufficiently-interesting thing, and in fact dislike the idea of doing so. I'm not entirely clear why. Possibly it's a slight feeling of resentment at the whatever-it-is for wasting the seconds of my life. There is an inexhaustible supply of things that would just love one's precious attention, and I feel a need to be harsh in culling them.)

[1] Dunn, Sarah. The Official Slacker Handbook. Abacus, 1994. ISBN 0-349-10591-X