Our local NWTF chapter has been putting on seminars since the early 1980's. They have evolved from being all-day affairs at the beginning to about 2 to 3 hours nowadays. Why is that?....because we found that 2-3 hours is about all most attendees can take without becoming antsy to get out of there. However, to really cover the topics that NEED to be covered about spring hunting, it takes that length of time.
Cover the basics,...hunting regulations, turkey hunter ethics, turkey biology that relates to why and how you hunt in the spring, equipment, finding birds to hunt, locating tactics, basic approach and setting up tactics, calling tools, their use, and BASIC calling techniques, tactics for roost hunting and daytime hunting, roosting techniques.
What you don't want to do is get too involved in all the various turkey sounds and how and when to use those. Stick to the basic yelp and cluck for a beginners seminar,....although you might very briefly demonstrate other calls so they have an idea of what they might hear in the woods. Getting too deep into the weeds with calling only confuses folks and takes way too much time. A quick story or two about actual hunts is good,...keeps their attention a bit and stirs excitement,...but don't do too much of that, either.
Bottom line is that if you are efficient in covering topics, you can cover pretty much all of the important stuff inside of three hours. The fly in the ointment is that, depending on how many attendees you have and their enthusiasm, you may be answering questions a lot,...which can drag the timeframe out considerably. ...But that is a good thing.
...And finally, if at all possible, have a bunch of giveaway stuff on hand and give away door prizes to attendees (give them each a ticket and draw prizes at various times during the event). Nothing like giving away free stuff to increase enthusiasm and participation.
Good luck,....turkeys hunting seminars are usually a blast.