When is the latest time one can make the intention (niyyah) for a recommended fast if the fasting conditions are not broken (e.g. eat, drink)?
If it is a recommended fast (mustahab) then you have all the way until moments before Maghrib to make your intention (niyyah) to fast for the sake of Allah, provided that you have not eaten or drank during the day and avoided the fast nullifiers.
[According to the opinion of Sayyid al-Sistani]