It's different for everyone.
I didn't meet my boyfriend's OSO until we had been together for 6 months. After 6 months, we were already starting to fall in love with each other, already very intimate with each other.
I actually felt hurt that I hadn't met her sooner. His past lovers had all met her and they didn't last as long as he and I had been together already. So why not introduce me, was there something wrong with me?
His reason was that she didn't have many friends and she tended to befriend the ladies that he brought home to meet her. Then, when the relationships ended soon after, so did the friendship with her. So once he realized how deeply he and I were involved with each other, he brought me home to meet her.
It went well but oh boy was I ever nervous!
To each their own...you have to determine your own boundaries.
Hope this helps!