I cannot answer for Bing, but for Google, the best thing to do is wait. Google will not recheck your sitemap every time they visit, but depending upon how often it has changed in the past, Google will assign a TTL (time to live) style value to your sitemap that dictates how often to check the sitemap for changes. If your sitemap changes often, then Google will check it more often. My sitemap changes between every one to two weeks. That is exactly how often Google checks for changes. However, I do warn you, if you try and refresh or resubmit your sitemap using Google Webmaster Tools, you will interrupt this process and unfortunately, you will end up waiting longer. Another warning. If your sitemap indicates that it has changed and Google fetches it and there is no change, it will not update the TTL for that file. So even if I regenerate my sitemap daily, but there is no actual change, then the modify date cannot be trusted and Google will only update the TTL according to when it actually sees a change. For a period, this may be largely unsettled, but will become more regular as Google takes more chances. Keep in mind that Google checks for additions or changes, but may ignore dates and variables that are associated with regenerating the sitemap. So, for example, if no pages are added, Google may see your sitemap as not having changed.