There's no one perfect time to sign, but as Q Da Fool told Adam22 on the No Jumper Podcast, there is a point where as an indie artist you feel like you've reached your limit. From there, the best thing to do is to start talks with a label that really knows and respects your work, which is exactly what Q did with Roc Nation.
"When I went to Roc Nation, it was all love," Q said. "Soon as I walked into Roc Nation, it was like they all did they homework on me. They already studied me. They was listening to my music. The [label] president was in there... Telling me that I'mma be the priority at Roc Nation. And he held his word. I love Roc Nation, bro, they changed my life. When I came home, I told myself I was gonna stay independent, but at one point I just felt like I hit a ceiling."
Now, when it comes time to sign to a label, the picture Q Da Fool paints is the perfect one. Roc Nation listened to his music, studied and respected his career up until that point, promised him priority at the label, and brought in the president of the label, Michael Yormark, to show that they mean business. All of these things combined make for the right environment to transition from independent status to a major label deal.
Staying independent has a lot of nobility behind it, but once you feel that you are out of resources and have done all that you can as an indie artist, the next step is to find the right label home for yourself. If you look for a situation similar to the one Q Da Fool found with Roc Nation, then you will be in a better position than most.