A lot of times, it depends on the type of game. In the past, I've seen people in PB-based games write 500+ word journal entries about their day, thoughts about happenings IC, etc. In fandom games, I've seen as little as one or two sentences. Depending on the character and what they're doing at the time depends on what they've written.
I know for me, I've had long journal entries, but I've also had those one to two liner entries. I'll PM you some journals that have entries. I think, at this point, they're mostly fandom games (Harry Potter or Marvel/X-Men), but hopefully they'll be helpful. :)