I'm going on a holiday, with my 2008 BMW 116i (122HP). The redline is at about 6500rpm, but i don't know exactly. Driving at highway speeds, the engine revs are around 3500rpm. Is this safe for the engine for a longer time?
This is a normal rpm range for a gasoline engine. Especially the smaller non-turbo engines need to run at higher rpm's in order to have adequate performance for highway speeds. You do not have to worry about the safety of the engine. These modern engines are thoroughly tested by the manufacturer in far more extreme conditions than you will encounter. They can endure this all day every day.
Let's start from the physics.
According to physics, force is mass times acceleration.
Acceleration, on the other hand, is velocity divided by time.
If the engine is spinning faster, velocity is greater, and the time to change it is shorter. This means acceleration is a quadratic function of the rotational speed.
Thus, force is also a quadratic function of the rotational speed.
Your engine can withstand the required forces at 6500 RPM.
(3500 / 6500)^2 = approximately 0.29
This means you are experiencing forces of only 29% what the engine is capable of. (There might be additional elastic deformation of material at high speeds, causing additional forces, so the 29% is an upper limit.)
I would say based on this your engine will have a very healthy and long life at 3500 RPM.
Normally your Engine is fine as long as you don´t rev it as long as it is cold. Always make sure to drive in low rpm´s for the first 5-10 minutes!
Other than that, the manufacturer has limited the rpm´s to a safe range that is well before any problems arise. There is an electronic rev-limiter so you can´t really over-rev.
That said, every revolution will amount to a certain wear. So from a longevity-perspective it is advisable to always drive in the highest gear possible if you are just cruising. Also from a fuel economy standpoint that is usually the best option.
Exception is when you want to accelerate fast. There it is advisable to use almost your whole rpm-band, because power=torque*rpm. You will feel what I men if you accelerate till the end of the rpm-band. Compared to the engines life, those acceleration don´t amount to much - at least if you are not a racing driver.