On a scale of practical to emotional:
When it's fully depreciated and routine replacement is factored into your business plan.
When it becomes too unreliable to get jobs done as scheduled. (Parts and repairs are rarely going to cost enough to justify a new unit, but unreliability will.)
You can afford it and you want a new truck.
When you fear that your customers know the type of truckmount you use is not the biggest, newest, or contains less stainless steel than your competitor's truck.
If you're unsure of your manhood and think a shiny new (make it REAL BIG if you're really insecure) truckmount will help.