That seems like a very long time to connect with a character. Do you mean 15 pages in?
I don't know about number of pages per say, because using the webcomic medium is so diverse and how people tell stories varies so much. For example, gag-a-day clips like Sarah's Scribbles, I can relate almost instantly. A single gag has drawn me in.
For more long form comics, some comics are faster to read than others (Like when they are vertical scroll like on Webtoons), then its faster to get into it and relate depending on the pacing of the story. For example, Let's Play, I cared about the characters by the end of the first episode.
Some comics it just takes for ever for you to get invested in the characters for some reason. I usually give a comic about 10 mins of reading time to see if I'm into it or not. If I have to read for longer than that and I still don't connect, I mostly give up and move onto something else.