According to Shia Fiqh the criteria for how long and more precisely when the parents MUST pay for their children life and comfort and even similarly when the children MUST pay for their parents’ life and comfort is that they (the children or their parents) cannot earn their needs and need help for them. That is before I go to work and my parents let me study, no matter if I have married or not, my parents if can must help me. They can of course ban me from studying and instead force me to work, then they can stop giving me salary. Similarly if my parents get old with no salary it will be my duty to give them salary as much as I can and suffice for them. Of course my sisters if have no work (so own no money until their husbands give them some) they can help only if they own any money otherwise they cannot help their parents without their husbands' permission.
See e.g. here as reference, the webpage is in Persian and the users with blue usernames are Shia scholars.
About the marriage although it is good that the parents help their children find husband or wife, and similarly it is encouraged that the children help their (single) parents find husband or wife, but it is not obligatory for any of them to do so unless there is a necessity, e.g. marriage is not obligatory but only encouraged but there may arise situations that it will become obligatory, then helping the one who needs to marry can be obligatory, either for the parents or the children depending on the issue.
See e.g. here as a reference, the webpage is available in Persian and Arabic, and the answer is given by scholars and also contains a Fatwa.
and last but by no means least, fathers have Wilayah over their sons and Wilayah of fathers of fathers are always greater, they can make a Mustahab or Makruh to turn into a Wajib or Haram, although they are encouraged never to use this authority of them unless necessary since they will be questioned for how much trouble they have produced for their children at the Day of Judgment. For example if I want to marry a girl and my father simply says "no!", even if that girl is perfect in every respect marriage with her would be Haram for me unless my father or his father allow me again. Similarly if I don't want to marry at all and my father commands to me to marry that special girl I have no way other than to obey. I have personally checked this law of Allah, Allah will put the best for the children in obedience of their parents as long as this doesn't include committing any sin, even if the father's command is wrong! Him being wrong will be bad for himself at the Day but anyway the goods for the children is in obeying him in Dunya. However, all these have some exceptions, for example if a girl wants to marry a guy that is proper for her in every respect, she needs to marry and also likes to marry with that guy, but the father of the girl doesn't let her so and there is no other guy at the time that she can marry with, then irrespective of her father she can marry with that guy and of course she would be responsible for her choice and her father will be responsible for him banning his daughter from that marriage and pushing her into mental and maybe bodily troubles. Therefore, Fatwa can change from case to case and you may like to investigate more around your special case.