An important overlooked factor is the warming up of the account, regardless of age.
You should start slow and increase DM's progressively within the first two weeks of usage.
If you notice any rate limiting message rest the account for at least a few good hours or even that whole day.
Note down when you started to get limited, and try to spread the messages within the day so they're not posted too fast.
To directly answer your question, a warmed up 6 months old account could go with 30-40 DM's/day without being affected.
If your messages are not making people report your account, then you'll have better chances of not being blocked/banned.