I know that glycerol stocks are typically kept in a -80 °C freezer, however there are some people who do not have access to such equipment. How long would you be able to keep a glycerol stock at around -20 °C (typical household freezer temperature). I would like to create a new stock before anything goes amiss.
While I believe that it is possible to keep a bacterial culture viable while frozen in glycerol stock at around -20 °C, as attested to by WYSIWYG, it is important to note that your "typical household freezer" may not be suitable for such a purpose.
This is because household freezers usually have an "auto-defrost" function. This makes it temporarily thaw out to remove the frost that condenses from atmospheric moisture. If storing sensitive materials such as bacterial/cell culture frozen stocks, this would lead to repeated freeze-thaw cycles which would damage the viability of the stocks.
For that reason, almost all laboratory freezers do not have that function, and instead need to be manually defrosted routinely.