How long should i keep a Secured Credit Card?

Im getting ready to open two secured credit cards to start building my credit, but eventually i want to get an unsecured credit card. In about a year and a half from now im going to start looking for a house.

I read that having a long existing account helps, so should i keep the secured credit cards until i can get a mortgage, or can i apply for an unsecured card a few months after using the secured card?

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