It seems that they will start by imposing this ban on Africa and South America, due to new variants. That will give them time to get the system operating. Then add other countries. How long it will take to get things started is to be seen. Hopefully, they have already prepared a specification for this (but I would not bet on it!) and sounded out airport hotels but, many of those hotels were mothballed and it may take a couple of weeks to get staff back, food and supplies, security - if the lower floors have fully opening windows they cannot use those rooms and so forth.
They will do this and I would expect the restrictions to be in place for at least three months.