How long do proxies last

If I were to buy a pack, how long would 1 proxie last me?

If you would inform yourself before asking you'd see that your question makes absolutely no sense.
There is no such thing as a general "proxy" lifetime as there are uncountable places where they run

Botnet proxies last until the hacked computer is cleared or rebooted.
public proxies last between minutes and months, usually shorter than longer.
private proxies last as long as you stay subscribed.

You can buy cheap VPS and buy 1 ip as low as 1$ per month.
Hoowever some VPS plans limited to ip count some offer 10 some less than 1o