There is no perfect time period that decides whether or not you and your partner will stay together forever. Things happen, good or bad that will always alter, harm, or strengthen whether a marriage will work in the long term, but in the dating phase, maybe put into consideration how long you've been with the other person and what you have and have not gone through, talked about, lived through, or accomplished, before you propose or say yes.
Dating someone 6 months of less
There is a reason marriages tend to fail the most for those dating 6 months or less. This is the honeymoon phase for just about every couple where everything is new, fresh, and intense. You're lusting after the other person constantly. Sex is amazing and constant. Your partner is up to hang out with you and do everything with you every time you call. Your partner is on his or her best behavior. Fighting is non-existent or not major. You don't really know their family, they don't really know yours and that dynamic on your relationship. Maybe you haven't gone through the holidays with them or know much more about them then surface things everyone else can look up on facebook and know. The problem is having and holding on to the expectation that everything from the moment you say yes or propose is going to be just like that every single day of your marriage. You haven't spent much time probing and getting to know your partner above the surface level either because you didn't want to or because they never let you in any deeper then a few random tidbits about them that don't amount to much. The only people in this category I wouldn't worry too much about would be people who get married much later in life, say in their 40s and beyond. Mostly because these people have been through life, have had the kids, have their professional lives and houses already sorted out, have had the adventures and armed with that knowledge and having learned from past mistakes, tend to know what they want and who they are, and don't waste time BS'ing a partner, but opt to lay their cards out on the table from the get go.
Dating someone for one to three years
These couples, for the most part, have taken some real time to get to know one another beyond just the surface. You've had or are having those real life conversations that no one likes to talk about in public. They know your past, your secrets, when their mother's birthday is, and they have perhaps lived with you for a while so you've gotten to see what it's like to be in each others lives not just on a few dates, but on the daily. Typically your families know each other for better or worse, and you have been fully immersed and integrated into each others lives. You've had a few struggles, have been through some things, have made your decisions to have kids, not have kids, live in the city, live in the suburbs, etc. The more successful couples of this bunch don't put any pressure on a partner to either say yes, or to propose. They've talked about marriage, decided they wanted it to happen, and if/when it does, they can be confident with their yes.
Dating someone longer then 3 years
This type of dating is good for those who started to get to know each other when they were young, like in middle school, high school, or even college. Perhaps you knew at the time you met you were either legally too young to marry or you knew you were too young to settle down. These couples have a need to experience more out of life, to accomplish more things on their own, to really question sometimes if getting married again, after say a failed marriage, is for them. Often times, even the patient can become impatient at waiting for a partner to make up their minds. They may start to pressure you or you them because you feel like what else do they need to know about you or why don't the love, trust, or want to marry you...at this point. There are also the long suffering couples who just continue to make a go of it until one just proposes and they get married, but both knew it was a bad idea given their history, in the first place. Staying together for a long time with no end goal or decision to just continue to date, in site often leads to quick divorces once the rings are on. Indecisiveness, resentment, pressure, long term bad relationships, and overall dissatissfaction that one "wasted their life" waiting so long for something they thought would be great to happen, tend to kill these relationships.