Drum Printer Scx4521f How Soon Should Laser Printer Drum Be Changed?

How soon should laser printer drum be changed? - drum printer scx4521f

A week ago I received a message on my Brother laser printer, a "Change Drum" coming soon ", so I ordered a drum line that have just arrived. These things expensive, so it will not change until the previous use. I get the message, but appeared to hundreds of pages of text message printing, and the print looks good. When will I know it is time to forever change the drum, and nothing can be damaged if you wait too long? Thank you.