Milloin is a temporal form of 'mikä', basically means 'when'. Koska means also 'when' but also has a meaning as subordinating conjunction 'because, since'. Monelta comes from moni =many, -lta is used with hours to express AT this hour. Kello kolmelta, neljältä, puoli viideltä. With times it can only refer to hours because of this. Mihin aikaan means basically 'to which time', it refers to time of day so it can be anything between 00:00-23:59. Also it means 'in what time' ='how fast'. Mihin aikaan juokset maratonin? -4h, 20m

Sorry, milloin, but not million.