So ill do a design then before i start ill look at the preview it will tell me it will take about 48 min to complete but when i press start it actually takes my machine ± 3 hours to complete…
So now i just want to know …am i doing something wrong or do i have my settings wrong or is a normal thing ?
The simulation will copy the movements of the engraver, but not its speed. The preview time is usually inaccurate for this reason, but a difference of that caliber means it’s not set up properly. If you go to edit → device settings → additional settings you can configure the simulation’s speed settings. Clicking read from controller should give more accurate results.
Trying to get it 100% accurate is a fool’s errand as there are some aspects it wont account for.
After you “read from controller”, you can also edit those entries manually to further refine the calculations.
My machine uses a non-standard GRBL config and doesn’t properly report max speeds and accelerations to LB (not LB’s fault), so I had to adjust pretty much every entry by hand. It’s still not perfect, but it’s typically within a few percent.
Next to the “read from controller” thing, I’d like to add that the laser has physical limits and LightBurn doesn’t 
Even if the firmware allows to run at 50,000mm/min and the manufacturer tells you so, it doesn’t mean that the laser mechanics can move that fast. Another limitation is the serial data transmission. Stay below 10,000mm/min (better below 6000) and you should be closer to the real execution time.