how often do i change the water of a 1 gallon tank w/ a filter?

i used to have my betta in a smaller bowl & changed the water every week. now i dont know how often i should change the water? and clean the actual tank? (because it has a filter?)

another thing is that i have some tiny rocks in the tank and one fake leaf in it as well. do i need to take this out and clean it all the time too?

how often do i change the filter?

thanks so much for your answers and help!