I've never planned a trip to Europe and no posts on this trip which leads me to think I'm quite possible crazy, so now I'm just checking...I get quite bored on trips so I like to only stay a couple of days anywhere & move on. So can I do 2 days in Paris, 2 days in Rome, daytrip to Venice and get to Santorini 1-2 days or is this just too much to do to enjoy it?

Not doable if you want to see anything.

No way.

Choose one:
Paris with daytrips (or maybe Paris/Amsterdam or Paris/London)

Rome and Venice

Athens, Santorini and maybe 1 other island.

so how many days are required if i wanted to make all 3 in one trip?

"2 days in Rome, daytrip to Venice"
Venice is 4 and a half hours from Rome by train. I don't think it's feasible to do a "daytrip" to Venice.

between packing and unpacking plus travel - this would give you practically no time at any of your destinations.
My husband and I are also "do-ers" while on vacation. We like to see a lot & do a lot while on vacation, and leave practically no time for relaxing. This said, on seperate trips we stayed in Paris for 6 days and Rome for 3 days and were not bored one bit. We probably could have stayed in Rome an extra 3 days and stayed occupied.
Dont underestimate these destinations. They are not world-famous for nothing - they are full of fascinating architecture, museums, monuments, cafes, food, culture, etc.

With 8 days I would either....
go to Paris for 6 days (with a Versailles daytrip or possible sidetrip to london)
Or I would go to Italy and split my time between Rome and Venice.

You aren't crazy to want to pack in as much as you can. Many people are tempted to hit as many locations as possible when they travel so far to get to a destination.

However, with your current plan you'll spend more time unpacking, repacking, and traveling between cities than seeing the sites and taking in the flavor of the locale. You'll be sacrificing quality for quantity. Don't forget that jetlag will likely affect your first couple of days.

My first choice would be to stay in Paris 4-5 days then hit the country (Normandy or Provence) for 3-4 days. If you chose to stick with Paris and Rome I'd do no less than 4 days in each city. Save Santorini for another trip.

If you're flying home from Santorini and don't have to backtrack then you could do this trip in 11-12 days at the pace you say you enjoy. At a guess the 'average' person would need 2 weeks or more.

Consider 3 days in Paris, 3 days in Rome, 2 days in Venice (a large chunk of which will be travel time from Rome and to Santorini) and 3 days in Santorini

Just wanted to add that I agree with other posters but the OP did ask how long it would take to see all these places (briefly!)

It would take a ton of time just to get from place to place. Plus you will never get bored in Europe! There is way too much to see. Stick to 1 country and do 2 cities, maybe Venice and Rome, or do Paris and somewhere in the country. Or Santorini, Athens and another island. I have dont Athens / Santorini with a stop over in London on the way, but I did not need to see a lot in London because I have spent a lot of time there.

That is just too much jumping around. You will spend your whole trip in planes, trains and automobiles, that is not a way to spend your time wisely.

For that particular itinerary, if you were really to do it justice, you would need 5 days Paris, 5 days Rome and 3-4 days Santorini and 3 days Athens. It just does not make sense.