We are considering a connection in Delhi (DEL). Coming in on Virgin Atlantic (from LHR), continuing on Spicejet to BKK on separate tickets. As far as I can tell both airlines operate out of Terminal 3. I assume we can't connect airside and will need to go through immigration & customs to check in with SpiceJet. We'll need (and can get) proper visa for this.
How much minimum time should be budget? Typically I would budget 4 hours for something like this (2 hours actual transfer time plus 2 hours of margin for delays etc) However, I've never been to India or Delhi, so I would like to get an opinion from travelers that have been there whether this is in the right ballpark.