About how long is the tutorial?
As long as you want. There is no mandatory tutorial IIRC. Once you start the actual game there is no handholding*.
There are some training missions in the main menu that are worth doing at some point though!
If you just want the basics, it's probably 20 minutes to do the take-off, travel and landing tutorials and maybe 30 minutes to try some combat tutorials. The standard newbie advice is not to worry if you can't do the advanced combat missions. The tutorials are essentially a safe space to die without it costing anything. The enemy AI has been dramatically improved since launch, so the main lesson to learn is to never attack 'wings' of more than 1 ship unless you know what you're doing. Getting outnumbered is bad, even if they are in crappy ships.
IMO it's best to do the basic training missions then start the game. Re-run the training missions or use google if you forget how something works. My google history is full of stuff like "Elite unknown fragment where?", "Elite wave scanner how to use?" and "Elite can I kill Marco Qwent?"
If you have time or get stuck and need help, this playlist has some thorough tutorials, though they are a bit slow and tedious:
https://www.youtube.com/playlist?list=PLmX89YRYfWK3QPK8tkUsiXVYqPTdJ4z07
The only fundamental rule is "Never fly unless you can afford the rebuy!"
If you die, you have to pay 5% of the ship cost and 5% of any upgrade costs (but the starter ship is free). If you can't afford it, you lose the upgrades and if you can't afford the basic ship cost then you get sent back to a crappy sidewinder starter ship.
It sounds a lot harsher than it really is, since player deaths should be rare unless you're a very combat-heavy player. You can check the insurance rebuy cost on the right-side panel somewhere. Note that insurance is automatic - you don't have to buy a policy or anything, unlike Star Citizen. What Elite calls 'insurance rebuy' is basically a 5% excess/deductible.
The archetypal newb error is to spend all your money on a fancy new ship, then crash it into the dock (because it handles differently) and then be unable to afford the rebuy insurance cost.
*Actually, the first time you ever launch there might be some forced "pre-flight checks" that make sure you know the basic controls. This takes about a minute to complete and is never repeated.
