3.0 L head bolts are M10 x 110 length. From memory, the engineering rule of thumb is two or three times the shaft diameter for the threaded engagement (for something with lots of pressure like a head bolt. Five turns for minimum engagement, ten is better, I can't remember any more beyond this (too may years since university).

You could consult to see what an engineering manual will give you but I'd expect the longest Timecert (actually you now need Big Sert as the block was previously Helicoiled) for an M10 bolt to be 20 mm or 30 mm.

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.