The answer to 'frequency of mulch' is that most experts recommend applying a fresh layer of mulch once or twice per year, typically in the spring and/or fall. Mulch helps retain moisture, suppress weeds, and insulate plant roots, but it breaks down over time and needs to be replenished.
The ideal depth for mulch is 2-4 inches, and you should spread it evenly around your plants and trees, leaving a small gap around the stems and trunks. Applying too much mulch can actually suffocate plants, so it's important not to overdo it.