Laxatives can be of multiple types - from simple dietary roughage to some OTC mineral oils to prescription intestinal stimulants, there are simply a wide range of substances that can loosen stools and increase bowel movements.
When should one take a laxative? I would say everyday. Dietary fibres are very good laxatives in normal individuals. Any diet should include plenty of dietary fibres. They also reduce the risk of colorectal carcinomas and reduce cholesterol and are all the more reasons to include in the diet - especially in the western population. For someone who follows a regular food habit and is generally healthy there won't be any reason to take anything else to increase bowel movements. So in short, dietary fibres(roughage) should be your everyday laxative.
If you are acutely constipated for some reason, and is making you miserable, and do not have any other symptoms(such as vomiting, abdominal pain, blood in stool, etc) then you can take an over the counter laxative for one day and see if it resolves the issue. That can be simple mineral oil, castor oil, Isapghula, etc. If a single dose of any of those does not improve your costipation, then you should consult a doctor. The doctor can look at the cause of constipation and prescribe a medicine for you appropriately for a short duration of time. Some people who take certain pills (opiod narcotics for example) are likely to be chronically constipated. In that case one may have to use laxatives for a long duration of time, but that decision is taken by a doctor.
The reason why it is not recommended to take a laxative for a long duration of time without a good reason is that some laxatives reduce cause colonic tissues to wear out over time and make the patient permanently consipated(laxative gut) There is also the risk of reduced nutrient absorption, fluid and electrolyte imbalance, intestinal paralysis, irritable bowel syndrome, factitious diarrhea, etc. So chronic use of stimulant laxatives should be avoided whenever possible.