Well basically 160 km (about 100 miles) at 20 km/h takes 8 hours.

160 km at 25 km/h takes 6h25m or so.

160 km at 30 km/h takes about 5h20m.

I used a calculator to get to these figures.

So say you ride with a friend, drafting each other in a headwind, taking it easy, side to side, when it's easier, and are both decent riders with some endurance, you could maybe do this at 25 km/h, so 6h30, plus you stop at SAG (say 3X20?), so 7h30, plus a few 5 minutes stretching breaks? So 8 hours total would be safe.

But of course that's a totally hypothetic scenario. It really depends on you.