I agree with XML-Sitemaps Support. If you have a large site, more than 500 pages or so, once or twice a week should be fine. Most search engines, such as google, will only crawl like 500 pages at a time.
But from my experience as an SEO, If you make constant changes on your site, adding new products, changing descriptions or prices, categories where items are displayed, things like that, you should do more often.
If you use some type of category system on your site (most do today) and you move things around a lot in different categories, that effects search engines. IE, if you have a category set up for items on sale each week, say google crawls and adds them to its index. The next time it might try and crawl those items/pages, they will return as unfound pages if you have removed them from being on sale. There is more to it than that, but just a quick thought on it.
If you make little changes to your website, then you will fine running sitemap generator much less often, just my thoughts on the subject