There isn't a magic number of minutes of exercise your dog needs before they can succeed at home alone training.
And importantly, the goal isn't to exhaust your dog so they'll sleep through your absence.
You want to make sure a dog's physical and mental needs are being met generally, because a dog who is under-stimulated, frustrated or bursting with energy may find it much harder to settle. But exercise alone doesn't treat separation anxiety.
So, how do you know if you're getting the balance about right?
Look at your individual dog, rather than a generic exercise formula. Their needs will vary depending on their age, breed, health, personality and what they actually enjoy.
Ideally, across their day and week, your dog gets opportunities to:
Move their body in ways appropriate for their age and physical health.
Sniff and explore, rather than every walk being about covering distance.
Use their brain, through things like training, searching, problem-solving and exploring new environments.
Do normal dog things they enjoy, such as chewing, foraging, digging, playing / social interaction with other dogs and their people, or investigating smells.
Have some choice and agency over what they do.
Rest and sleep properly. More activity isn't always better.
One of the things I look for is what happens after activity. Can the dog come home, decompress and settle? Or are they still pacing around, demanding constant interaction and struggling to switch off?
And be careful with the idea of giving your dog a huge walk or an intense game of, for example, chasing after a ball immediately before separation training to "wear them out". A physically tired dog can still be anxious, and highly stimulating exercise can leave some dogs more aroused, not calmer.
For home alone training, I'd much rather see a dog whose needs have been appropriately met and who is comfortable, regulated and ready to rest than one who has simply been exercised to exhaustion.
And if your dog is getting appropriate physical exercise, sniffing, enrichment, opportunities to use their brain and plenty of rest, but still becomes distressed when you leave, don't assume they need even more exercise.
That distress may have very little to do with how much energy they have left in the tank.
A well-exercised dog can still have separation anxiety.