This update is important for several reasons:
It is being rolled out gradually, so it may not be immediately apparent if you have been affected. It may take weeks or even months before websites start dropping in their rankings so it is essential that your website is mobile friendly as soon as possible to prevent a fall at a moment’s notice. Google hasn’t mentioned when exactly the update will occur outside of them stating it would “start rolling out” at the beginning of May.
The algorithm assesses a website on a page-by-page signal, meaning it will take time for it to analyse an entire website. Depending on how quick Google can crawl and index all the pages on a website, the impact may be even slower to show-up.
If your site is already fully mobile friendly, then good news - Google have said that you will not be impacted by the update.
However, if your website is not mobile friendly then now is the time to update as you will only prolong the inevitable tumble down the rankings into oblivion.