I'm doing some work on the engine which requires me to remove the fuel lines in the engine bay near the injectors. I don't have time to finish the job in 1 day, so the fuel lines will remain disconnected for about 5 days. I have approx 30L of fuel left in the tank. Will leaving the lines disconnected cause petrol in my tank to evaporate by a considerable amount (such that I won't be able to start the car)?
No. It will not evaporate by much at all. I would say it would be nearly unnoticeable. If you are concerned stuff a decently fitting clean bolt in the lines or something to plug it up like a rubber cork. This will prevent spillage from fuel that is in the lines.
Not to mention the fuel would probably need to get through some filters and pumps too.
The fuel in the pipes might evaporate which might mean you will need to let the fuel pump build pressure first and in rare cases you may need to prime the system but I think you will be fine. If it doesn't start immediately after hooking it back up make sure your fuel pump is pumping and give it a few seconds usually it will stop once pressure is high enough.