The cause of 'Dosh' in the Kundli is due to the negative positions of the planets. However, when a planet is in its negative state or if the ascendant or the zodiac sign is under the direct influence of the negative planet then there are high chances of these situations of negative influence or effects of these planets. Dosha may also be associated with the individual's present birth as well as their previous birth. When there is a 'Dosh' in the horoscope of the person, instead of receiving the auspicious results of the some planets in the said state, the impacts would be negative which can be quite depressing.
This undoubtedly depends on the kind of 'Dosh' in the horoscope. Different kinds of 'Dosh' can last for a short period as well as for long periods. If the 'Dosha' in one's horoscope are left unattended and remedies for it are not done the effects may prevail in one's life for a long time.
As an example, in order to get rid of 'Manglik Dosha', the person has to marry a 'Manglik' only or else has to perform other astrological remedies advised by expert astrologers. Likewise, if there is a 'Dosh' related to Saturn/Shani, its duration is also considered to be for a long time.