The party levels after 13.33 encounters. So says the 3rd edition Dungeons & Dragons rules.
If you assume the party levels after two "adventures", and each adventure has just one barely connected wandering encounter, you're looking at 5.5 encounters per adventure.
Needless to say, 3e doesn't support the epic 30 encounters per adventure, 5 adventures per level games of the past.
Now, if you're going to give XP for other stuff (roleplaying, writing a campaign journal, whatever), you have to give enough that it's worth something. Getting a bonus handful here and there isn't worth it. What's enough? Beats me. To pull a number out of the air, let's say 1 encounter worth per adventure.
Down to 4.5 encounters per adventure.
Of course, as the characters travel from adventure location to adventure location, let's throw in a completely unrelated wandering encounter or two. Let's say one per adventure.
Down to 3.5 encounters per adventure.
Gee, those adventures are getting short.
This is fine, but you need to be aware of what adding more XP to the game does. Characters advance faster. Alot faster. Levels are cheap in 3e.
Relatedly, if you're effectively reducing the number of monsters per level (by giving them plot/role-playing XP), you're reducing the amount of loot they're getting. Remember to compensate.