Oktoberfest - How Long to Spend?

There is more to Oktoberfest than the beer tents.
Adjacent to the tents is an area with rides and food, so you could spend time walking around there, too.

We visited the big fest three times, since we lived in Augsburg, about 50 miles west of Munich.
We always did a full day.

Loved going one year for the beginning of the Oktoberfest and watching the parade of all the beer wagons, with people dressed up in local attire.
When we did the Oktoberfest, we usually went earlier than most, arriving no later than 11am so we had lunch (get roasted chicken). The crowds are not as heavy as in the late afternoon and evening. It does get more festive with all the singing later in the day, so stay for that.

We never made reservations, but that was over 35 years ago. One thing. that I learned was drink your beer slowly. It is stronger than American beer. I suggest for the average person, don't go over four liters of beer.

Do not drink beer and try to do the rides, or your beer will revolt.

We did three days (and an extra quick stop):

1. Day to Wander, get our bearings, figure out which big tent we wanted to spend our time in later (they all have pretty distinct personalities)
2. Old Wiesn / Rides / Fair Food
3. Day in the Big Tent we identified that we liked the most (since you have to get there early to snag a spot at a table without reservations)

Oh, and then we cancelled our last morning breakfast before flying home to stop in for one more wiesnhendl (the rotisserie chicken). Don't sleep on the chicken.

I agree with others, there is plenty to do outside of the tents, and even the fest grounds. I have been three times, going back with a group of "first timers" in 2027. I usually go opening weekend and target about 4-5 days. The typical thing is usually:

Friday, just exploring Munich, hit a couple beer gardens/halls.
Saturday, the Brewers Parade, then we usually avoid the fest grounds. We have even headed out of town to someplace like Klosterbrauerei Andechs. The beer gardens can get pretty lively and fun in Munich on Saturday Night.
Sunday, most shops and things are closed, activities have ranged from beer gardens to hiking. Spent the day in Tegernsee once, took the cable car up to the top of the mountain, and hiked down, then some time at the very good brewery there, and finished the day at the sauna by the lakeshore.
Monday was the day we always hit the fest grounds, the crowds are minimal (for Oktoberfest) you can walk through the tents for a look, get seats in about any tent. Even in the evening you should be able to be seated.

Only one experience. One afternoon at the actual fair grounds was enough for me. It's like a giant state fair with rides and side shows and music etc. The tents were good for a beer and a chicken and if you want to join the drunken singing, We were there in a weekday, and it was very much family time. I believe it gets more boisterous in the evening. It was worth doing but not for multiple days. The rest of the city looked pretty normal.

We went to the actual Fest grounds three days - once on a Monday afternoon and then on a Tuesday and Wednesday evening. I don't do carnival/fair rides, so I skipped those although my partner and one of our friends did a roller coaster in the evening after a couple of liters of beers (they were fine).

I liken it to Vegas - walking around and people watching is fine, but honestly by the end of our third session on Wednesday, I was done.

Let me add that there is variety between the tents. Yes, every night the well-known big tents have 5-6 thousand people standing on the table benches singing American pop songs while holding a liter of beer. There are also 20 smaller tents that have a very different vibe. Last year we had one evening's dinner at the Heiner Entenbraterei. They specialize in roast duck, only seat a few hundred people and are very low key as compared to the big tents.

Where else do you plan to go? We also attend Stuttgart's Volksfest which takes place around the same time as Oktoberfest. It's about half the size of Oktoberfest and for me, has a someone less beer and more community feel to it.