it depends as per your keyword competitors and algorithm... if you are on 1st page of google then there is no need to change on-page till you get down from SERPs... everybody wants to come first.. so they used to check what have done by the 1st page result's website does..
there is no set time period for that. some pages/sites will not require updating that much. some will require frequent update. all depends on how your site demands.
If you are not getting results then change it in 3-6 months. As you will get to know about your standings in SERPs within the mentioned time period. Otherwise there is no need... Thanks
content is the king.. so if update daily the content is good and all SE like fresh content and once keeping update SE bots crawl the site daily some time many time in a day.. this is good for site ranking and reputation
updated and unique content is most well come in ever search engine..so udate daily if your have blog site..
When you doing your website SEO and your website ranking is not increasing in SERPs, so you can change your onpage of your website.