You're looking at your beds, and you say to yourself... "My landscaping needs to be refreshed"
Well then... re-mulching them will give them a nice clean look.
During my 15 years running a landscaping company in Nashville, TN, most of our of clients would re-mulch once a year. Some would ask for it twice a year (when budget was not a factor) in the spring and fall. Mulching in my opinion and experience is 50% aesthetic and 50% environmentally necessary for the health of your plants.
The new mulch really gives a clean uniform appearance to your landscape beds, however, if you don't care the overall appearance that much then you could defer adding mulch every 2 years. If all you care about is the environmental benefit for your plants then mulching every 2 years is fine.
Functionally speaking, mulch helps keep moisture where your plants need it most, at their base. It also protects the base of your plants from insect feeding which is critically important as nematode insects can wipe out an entire landscape right under your eyes.
So if you've decided that's its time to mulch, next question is how thick should you go?
Well, that depends on how often you are dressing the beds with mulch. If you are doing it once a year then 2-3 inches will be enough. Just apply enough to where you cant see any of the old mulch any more but no more than that. A pitch fork is great for applying the mulch and moving it around but be careful not to overdo it. If you place too much down around the base of the plants, (5-6") then this is harmful to the plant material and will caause you problems with fungus, and possibly even death of the plant. Plus, if you go thicker than necessary you are just wasting money in mulch, and unnecessary work. Laying and spreading mulch is kinda hard work!
If its been a couple of years since you have re-mulched, then you will need to go thicker, say 4-5". All of the mulch in your beds will biodegrade to bare soil in 3-4 years time. So the longer you wait between redressing the beds, the more mulch you will have to re-place. In some markets such as Atlanta Georgia, they predominately use pine straw and not much so these rules of thumb wont apply.
Take for example... Lets say my company installed an landscape job for a customer from scratch.
In this example, the initial installation of the project would take 30 bags of mulch. Then a year later, we came back and redressed the beds, that would take 10 bags. If the customer waited 2 years it would take 20 bags. If they waited 3 years or more, it would require then entire 30 bags again.
Hope this info helps you gauge how mulch mulch to apple on your re-mulching project.