From what I know, it really depends on the program that you are applying to. E.g. for Finance and Econs, some schools don't really look at your work experience (WE). For OB and strategy, some WE would be useful. Generally, WE is not as important in PhD application than in MBA application. Research experience, on the other hand, is more important in PhD app.
As to how many years of WE is helpful - there is not reliable stats on this as far as I know. However, you can consider working backward: If 30 y.o. is the critical age in applying to a PhD program, then (30 - current age) should be the MAXIMUM number of years of WE that you should get. I would say in most cases, 2 years of WE should be sufficient.
(Note: 30 is an arbitrary nuumber. There are people who get into a PhD program at 32, 34 and even 38. However, many have commented that beyond 30, the chance of getting into a program starts to decline, unless you have an extremely strong profile.)