Once a week is fine. These salamanders can get overweight quickly. Marbled Salamanders don't do much and live in a relatively low temperature range, so a little food goes a long way.

I have Tiger Salamanders, and sometimes I'll skip a feeding if they still look fat from the previous week's feeding. One thing I'd do if I were you is to add some vitamin/mineral supplement to part of the worm once a month. I do this after the salamander has grabbed the worm and sprinkle the supplement on the end of the worm hanging out of the salamander's mouth.

I don't thinkthey'd like the taste of the vitamins/minerals, but once the worm's in their mouth, they'll continue to eat it.

Tim

Aboreal Salamander I found in California a couple months ago:

Third Eye