Home Forums Hardware Mac Desktops & Notebooks When to replace imacs and Macbooks?

I have a 27" imac 12/09, and a Macbook air 11" 2012. They have had good runs but are both having issues. The desktop needs more Ram and a new Hardrive (I would get an SSD). The laptop need a new hardrive as well as the 65GB is now too small.

I could keep investing in these two machines, but would like to ask: When does one decide to upgrade desktops and laptops in general-- even though you could just keep replacing stuff?