How long should I wait to replace a cheap used iPhone without upsetting my dad?

My dad went through a lot of trouble to get me a used iPhone for $50 so I do not want to annoy him by getting a new phone right away. I thought I could get a new iPhone in 3-7 months, that is a polite time to get a new iPhone if you do not like your old one right?

How long should I wait to replace a cheap used iPhone without upsetting my dad?
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