It's not necessary for you to be present for the inspection, but it is recommended. By following the home inspector around the house, by observing and asking questions, you will learn a great deal about the condition of the home, how its systems work, and how to maintain it. You will also find the written report easier to understand if you've seen the property first-hand through the inspector's eyes. The seller is welcome to attend the inspection although they should understand that the inspector is working for the buyer. Consequently, comments will be made in a critical sense about flaws, defects and problems in the house that could be upsetting to some sellers.
Time is dependent on the house, the inspector and the client.
Inspections can take from an hour to over three hours. The inspector you select can best estimate the time when you schedule the inspection. No house is perfect. If the inspector finds problems, it doesn't necessarily mean you shouldn't buy the house, only that you will know in advance what to expect. A seller may be flexible with the purchase price or contract terms if major problems are found.