The issue is not when you say it but HOW you say it. The increasing intensity of 'Thank you' is like this: Thanks. Thank you. Thank you very much. Thank you very much indeed. So, obviously, the more grateful you are, the 'bigger' the thank you. You say 'Thanks' for a small favour, and 'Thank you very much indeed' for a very big one. But the important thing is to say 'Thank you very much indeed' sincerely. If you get the intonation wrong, and you say it in an insincere way, it can be misunderstood. It can sound sarcastic and rude if you don't say it with the right degree of sincerity and accompanying eye contact and pleasant facial expression. So, by all means use this phrase if it's about something that you are very grateful for - but do make sure that it sounds genuine when you say it.

Thank you very much indeed Su.Ki.!