How much time at Keukenhof Gardens?

Your timing is good. Do the gardens as near to mid April as your itinerary allows. I would think that you need about four hours in the gardens and add transport to that. I'm not sure that this is possible but, in a perfect world, you could visit the Aalsmeer Flower Market (it is very near the airport) early in the morning (they open before 7 AM) and then continue on to Keukenhof. Keukenhof is exceptional but the flower market is also quite remarkable (it would take about 1.5 hours plus transport.) We didn't have a tour. I dont think you need a guide for either place. Enjoy.

I spent the better part of a day there, but I really like gardens. I don't think an organized tour is necessary. I took the train to Leiden where I switched to the bus and it was all quite straightforward. The website describes the various transportation options.

Once in the gardens you need 5-6 hours to relax and enjoy the immense beauty of the Keukenhof Gardens. While at the gardens we purchased 90 tulip bulbs in april 09 which was sent in the mail the following oct. Last spring the tulips bloomed and was a nice reminder of the gardens. This spring the same tulips are starting to bloom and this beautifies our gardens.

I'm not sure how long it takes to get there from Amsterdam. But for actually walking around, how much time depends on when you arrive and how long you like to linger . If you arrive near opening time, you can expect to see everything in 2-3 hours without lingering. By lunch time, the place gets pretty crowded, especially the indoor exhibits- add an extra hour or two.

I don't know about Holland, but right now, just about all the bulb plants are already up and blooming in the Frankfurt area. Daffodils have exploded through all the parks! I would check with the Holland folks over on Trip Advisor to see how long the flowers are in bloom there.