Mulch is best applied either in autumn or spring when you are cultivating the soil. (However, be careful not to apply at the same time as you add lime to balance acid soils, as these will react creating ammonia.)
Most good garden soils will benefit from about a bucketful of well-rotted organic matter per square metre each spring. More depleted soils may require up to four times this amount. On lighter soils, apply organic matter in spring so that nutrients aren't washed out by winter rains.