What I see at shows averages around 5mm. Still, there isn't going to a "standard" that applies to all slabs.
For some agates, a thinner slab is going to be better (iris agate, dense plume and moss, some turtleback, etc.). Thinner slabs are also going to work better for jewel and lapidary items that will be kept lightweight (earrings, tablet-top gems, etc.). I've seen some high-domed cabs that were heavy enough to cut through earlobes or seriously dent the back of a neck in a necklace - weight can be a consideration for the final use. Some rarer and hard-to-source materials have always been cut thinner (3-5mm even in larger slabs) to maximize yield from expensive rough, and little choice but to cut low-dome cabs from those. Lighter is better for most jewelry, though.
Some materials (optical phenomena such as bull's eyes, cat's eyes, stars, labradorescence, etc.) often do need higher domes to show best, and something 6-7mm works really well for those. Large, collector-size cabochons can also be easier to cut with a bit of extra thickness. Slabs that may be intended for carving and similar lapidary work can be very thick indeed.
So, you get to decide