It's ages since I last made a cake.
_________________________________________________________________________________________________________________________ The structure it's (been) + days / weeks / months / etc. + since is used to emphasise the length of time that has passed since a past event: It's been weeks since his last blog post. It's been a long time since I met him. How long has it been since they moved away?
In the sentences above, it's been stands for it has been (present perfect); however, in everyday speech, it is more common to use the present simple instead of the present perfect: It's weeks since his last blog post. It's a long time since I met him. How long is it since they moved away?