I think it would be more accurate to base these steps on events, rather than time in certain cases.
- Becoming exclusive: as soon as you think they are a good fit for you. This takes talking about how they approach relationships, communication, sex, disagreements, and compromise. Let's say after 3-4 dates or so, you should know if they are a good fit.
- Meeting parents: I'd say after about 6 months or so.
- Moving in together: My rule is: never move in together unless you've been dating constantly for 12 months. So if you date for 8 months, break up for a month, then date for 4 months, you are not ready to move in together. (Never put BOTH people on the lease. It's too much trouble and expense to kick one out then.)
- Engagement: I usually date 1 year, then live together at least a year before getting engaged. If the girl pressures you to get engaged, she is not the one.
- Marriage: this all depends on how well you know the girl and what she's like. You have to put aside your emotions and really think "Does this person have all the skill that a long-term relationship will need? Do they have good communication and problem resolution skills? Or do they get defensive when talking about a sensitive topic?"
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)