We recommend you get a good full groom at least every 3 months but there is benefit to doing a full groom every month if you can afford it. With Maltese you might find it is easiest to keep them in a nice puppy cut most of the year long. Maltese are especially prone to matting and gunk in the eyes that comes from allergies, dust and irritation. Eyes should be cleaned regularly and when you notice excessive gunk in the corners of the eyes. Not all Maltese gets the goobers in the corners but some do. I usually try to collect any larger goobers in a tissue or paper towel and then trim out anything that is excessively dry and gunky but I am very careful not to aim scissors at the eyes. I am careful to angle safety or round tipped grooming scissors up to an angle where they cannot even poke an eyeball and I hold the muzzle very still while angling the scissors away from the eyes and trimming out any bad hair and gunk. If your dog is struggling it is best to trust this step to a experienced groomer.
We do shave ours down often in warm weather, perhaps in spring and then during the summer and they look amazing! We allow them to grow a bit more hair during the winter when it is cold and often use dog sweaters or t-shirts during the fall and winter season. I have found it is best to allow an experienced pet salon take full care of the dog including trimming nails and expressing anal glands.
If you plan to show your dog, do not cut their hair, brush daily and seek professional grooming advice from a seasoned show groomer. Dogs who are show dogs usually keep their long hair in braids, pony tails or various paper wraps. I have heard once you cut their hair you may not be able to show them.
Grooming your dog should be done as often as possible!
Good resources for show grooming
Dog On Top
Learn to groom your maltese for show:
Similarly for a Yorkshire Terrier: