How can I estimate the duration of a long bicycle trip?

It actually worries me a little bit that you are planning such an extensive trip without the capability of estimating yourself how long it will take :)

Checking out estimates for other bicycle routes might help you, but my experience is that they are exactly what they are labeled: only estimates. To give you an answer with a higher confidence rate than rolling some dice, you have to fill out a few blanks with your personal conditions and preferences:

• What is your pace? Even with a moderate physical condition and some weight from your luggage, you should easily be able to keep 15-20 km/h on a flat route. If you calculate as low as 30 km/day, you shouldn't need to spend more than two hours a day on the bike. If your condition is well above average and you plan to spend more time on the bike, your daily leg may just as well be five or even ten times longer.

• How much time do you want to spend on the bike and how much time do you want to spend on interesting points along your route, meeting other people etc.

• How "weather sensible" are you? If the rain is pouring, will you stay in bed or still force yourself through the daily leg? After you've decided for a specific region and the time of year, historical weather databases should be able to give you an estimate on the expected weather.

• What are your accommodation requirements? At least in the more rural areas in northern Sweden and Norway, it may be difficult to find lodging along the shortest route, forcing you to choose a longer route than strictly required.

I am not sure if my personal experience is of much value here, but I made a longer bike tour in an oddly mixed group last year. The attendants ranged from a 23 year old sport science student on a rented e-bike to a 65 year old tweedy gentleman on a rusty 3-speed bike and we rode a part of the Weser-Radweg in Germany from Hannoversch Münden to Bremen (about 350km). The trip took us 21 hours (real "on-bike" riding time) over three days and arrived safely after the sport science student gave up on the last day and took his e-bike aboard a train for the last 60km or so. As you see, it is not at all easy to estimate.