Any amount of regen results in considerable losses...
An EV's motor is at best 80% efficient at converting electrical energy to mechanical output at the drive shaft. Then, there are some losses in the gear box too.
When regen kicks in, the EV motor becomes a generator, converting mechanical energy at the drive shaft back to electrical energy stored in the pack. However, most EV motors aren't as efficient at acting as a generator vs a motor.
In a perfect system, regen recovered energy would be the power originally drawn from the pack times the square of the motor's efficiency.
So, assuming perfectly symmetrical efficiency (unlikely), if 10 kWh of energy is drawn from the pack to accelerate a vehicle from 0 to 30 mph, then regen would at best put back 10 * 0.64 = 6.4 kWh back into the pack when decelerating from 30 mph to 0 mph.
But, EV motor's efficiency isn't linear at all speed either. Then there are the gearbox losses to consider...
I would guess that if you can get 50% regen efficiency average (so 70% motor and gearbox efficiency combined), that's pretty good.
I've had conversations with people who swear that they can go up a hill and come back down again and end their trip with "almost the same pack SOC". It's laughable that they think that's even close to possible.
Note that with cruise control, the car will do regen on downhill sections or when slowing down with adaptive cruise. If you want higher efficiency, you need to learn to coast in neutral at the right times. Doing that can definitely beat cruise control on longer trips, unless the highway is dead flat and you never change speed. Coasting downhill is super efficient, unless you go so fast that increased air drag cancels any gains.