To get the most out of your day, I would advise you to leave early so that you can reach Keukenhof gardens before the big crowds. Buy your tickets in advance so that you can also skip that possible line. Upon entering the park, there are maps of the park for you on which you can determine your route and have a bit of an idea about the size of the park. There are beautiful paths to follow and along the way, you will pass some pavilions with flower shows.
You can also take a trip with a whisper boat at the windmill in the park. Enjoy the quiet, typical Dutch landscape around Keukenhof gardens in an electrically powered whisper boat. The boat trip lasts 45 minutes and you can listen to an explanation about the Bollenstreek during the trip! It is best to buy tickets for this in advance so that you are sure of a place. Tickets for this trip are available via this link.
On average you will need 4 hours to see the entire park. If you go early, this gives you the opportunity to do another activity in the afternoon. You can rent a bicycle or, for example, pick fresh tulips yourself on the field. (Read further for more information).