How far you must see, how long it takes, and when it's already too late.
Before starting an overtake, you must see enough clear road ahead to: 1. Pass the vehicle ahead 2. Return to your lane at a safe distance 3. Do all this before oncoming traffic reaches you On rural roads this typically means 300–500 metres of clear sight.
You're at 90, car ahead at 70. Difference 20 km/h ≈ 5.5 m/s. Passing takes about 15 seconds + safety gap means about 20 seconds. In 20 seconds you cover 500 m, and oncoming at 90 km/h covers 500 m too. You need ~1 km of clear sight for a safe overtake in that situation.
The smaller the speed difference, the longer the time and distance you need. Overtaking a truck that's 5 km/h slower than you is more dangerous than one that's 30 km/h slower.
If you hesitate mid-overtake, brake and drop back behind. Forcing through when oncoming appears is the most common cause of fatal head-on rural crashes.