You will need to rest for a few days to regain your strength.
Whether or not she will ever regain her strength I cannot say.
You will need a few months to regain your strength.
They had no food left to help him regain his strength.
People did everything they could to help him regain his strength.
This was not enough for his body to regain its strength.
It might take days, perhaps weeks, for his body to regain its full strength.
It was the only way she could regain her strength.
It will be many years before we regain our strength.
Many such patients take nine months to a year to regain full strength.