Charge time 24 hours

Is it dangerous to charge battery 24-30 hours?

I have 40aH battery and my charger is 2A.

It doesn't matter how long a battery is charged. What matters is "over" charging. So, if it takes 30 hours at a low enough current, that's fine. If it takes 2 weeks at sufficiently low enough current, that's fine. As long as the cell isn't over charged.
However, with that said, if a battery is "never" getting charged and a long time has passed, there's something wrong. It's self-discharging.

40Ah battery charging at 2A should take about 20hours from full discharge to full charge. If the battery was 50% SoC and it still has taken 20 hours, then there's an issue somewhere. Either a bad connection or you have a self-discharging battery.

If its lead acid you can keep plugged in for 24 hours, once its fully charge the battery won't take any more amps. The only way to overcharge it is to raise the volts above 14.4 volts. At lower voltages you can keep them connected for months.

For lithium, its a good idea to unplug as soon as its fully charge. The bms will stop amps going into the battery to keep if from being overcharged, but what if the bms fails.

+1 to Korishan & jonyjoe's points.
- the charger should limit the final voltage to suit the battery or you'll overcharge it
- a smart charger will automatically switch to float mode when they reach "full"
- a lead acid battery will be a bit more tolerant of overcharging (gel cells will dry out)

If the charger is a simple one & doesn't limit the voltage then no, don't leave it connected.
You can fry either lithium or lead acid types....

Jony its the other way around. Lead acid need to rest in between. Lithium dont have to. La have lower resting voltage meanwhile lithium Will stay at full voltage with 0current