I would be totally uncomfortable with that answer.
Ideally, I would volunteer in both of my kids classes each week.
When DD was in 2nd grade both DP & I volunteered once a week in her class. Generally I was working one on one with kids and DP helped with math most often (it was a split class and he would help the youngers).
DD was in 3rd grade last year and the teacher only allowed one volunteer a day. At first I was a bit upset that both DP & I couldn't go in weekly (we really like to stay in touch with our kids AND show them we consider their education an important activity). Then she explained that if you are volunteering for her she will have actual *stuff* (like copying, or a project) to do. BUT, you were welcome to simply visit any time you wanted

Very happy about it and DD has the same teacher again this year.
DS was in Kinder last year and I wasn't assigned weekly, but ended up being an art helper and being "on call" so ended up there about weekly anyway. I'm not sure how his teacher will be this year.
I can't imagine a teacher not wanting more help! Even if you just are taking bulliten boards apart or setting up stuff for lamination, I really like seeing what my kids are doing. Also, it lets me get to know all of the kids in their classes.

At this age (6 (AS OF YESTERDAY!!!) & 8) I can't imagine only volunteering maybe once a year in their class (also, that just doesn't make sense to me--- how about art days, class parties, field trips, etc... those alone happen more than once a month).
If the teacher is set on this and you were otherwise happy with the class I would ask the librarian, music teacher, pe teacher, drama teacher, lunch supervisor, whatever if they would have volunteer opportunities.