That is why you have sitemaps, for new websites with no sitemaps we could start with a default frequency, say, once a day. If it changes more often than that you can increase it to multiple times a day and so on. We can also look for the type of website to determine a starting frequency — for example an e-commerce websites ought to be crawled more frequently because new products, reviews, ratings could be added every few minutes whereas a personal blog may stay the same for weeks altogether.