Short version: quite a lot.
The long version below is a rough attempt for me (a former scientist) to understand the problem and the rough nature of possible solutions.
It starts with the R factor - essentially the number of people that an infectious person passes the disease on to. The more people you infect, the faster the disease grows. If you manage to get to a situation where people infect less than 1 person on average, the disease will decline.
I've seen R factors for the UK of round about 3 in the early stages. That's the effective R factor - even then, before our governmenat finally bothered with a form of lockdown, a significant number of individuals and organisations were taking preventive measures. So the intrinsic R factor could be a bit higher, although 6-9 is the highest range I've seen.
As we've seen, the effective R factor depends on reactive measures and behaviour. So what sort of measures are necessary to get R below 1?
Reember, R is the number of people who can pass the disease on to. So you need to reduce that number from anything between 3 and 9 to below 1. Actually, you need to go a decent way beyond that to give yourself a margin of error. So I'm going to say that you need to reduce the likelihood of passing the infection of by a factor of 10, which basically means reducing direct contact by a factor of 10.
And reducing transmission by a number that large is quite a challenge. If you just carried on and tried to use face coverings and social distansing, I don't think you can get there - they might give you a factor of 2 or 3, which isn't enough.
What level of contact do people have with others? I'm going to try and estimate the number of person-hours of contact you generate in a week. Remember, this is before the pandemic.
- Work, you might have 40 hours a week in an office or building with 25 people, giving 1,000 contact-hours
- Commute, You might have 10 hours (1 hour per journey), but cross the paths of 100 people, giving another 1,000 contact-hours
- Play, a restaurant or bar might be 100 people or so for a couple of hours, but you have to get there and back, and might do it several times a week. And I'm ignoring massive events like theatre, gigs, festivals, sports. So that could be another 1,000 contact-hours
That gives 3,000 contact-hours per week, pretty much evenly spread across the 3 categories. And we want to reduce that by a factor of 10 to 300.
Even if you avoid public transport (which has other negative implications for the next major disaster of climate change heading our way) you don't get anywhere near. Carry on as before and drive to work and you only get from 3,000 to 2,000, nowhere near the 300 desired.
As far as I can see, the only way to even get close is to not go in to the workplace, which cuts out the commute part as well. And, in addition, cut down the play component and take all the face-covering and social-distancing precautions when you do.
I see no possible scenario in which widespread full-time return to the office can possibly be entertained as being safe.
Even if we use the idea of only some people going to the office 1 day a week, you're still using up a very significant portion of your contact budget.
Of course, there are those who do need to go to a workplace. There are those who do need to commute. That's even more reason why those who can work from home should do so, to give a bigger allowance to those who really need it.
You can, very roughly, estimate your own exposure. For example, I went out to Ely and had a guided walk with a group of a dozen people or so around the town today. Socially-distanced, including the travel and an outdoor lunch, that was something near 50 contact-hours.