which is the best way to increase icloud storage

Currently I am out of my limit of my free 5gb storage on icloud. I have been sent a message telling me to upgrade to 50gb for $1.49 a month. However, it doesnt say for how long you pay this for? I am a bit reluctant to give my credit details. Is there an easier way to do this and is there a limit of time for paying the $1.49?

thanks

Max