I won't speak about rules, but only my personnal use of the language. "Sûrement" and "Sans doute" (which litteraly translates into "Without a doubt") should carry a huge amount of certainty. BUT, generally, it is really common to say such words to express the uncertainty, like "Is he coming?" "Oui, sans doute" but in fact, there is a doubt. All I can advise you is to respect that Sûrement is stronger than Probablement (probably).
Hope it helps.
It's easy to blame DL when there are so many mistakes. But this is not one of them. The primary definition of "sûrement" is "most probably". When we look at the French word, we want its meaning to be what we think it must mean, but in fact, we ascribe a meaning that it doesn't have. Then we start playing with synonyms in English for the word we (mistakenly) associate with it. The whole range of words: surely, certainly, and even doubtless often don't convey anything like certainty.
As a former native English teacher, I cannot envisage a way in which the sentence: "In a long time, surely!" could possibly used in English.
We would normally say "A long time ago, surely!" or "Long ago, surely!" ... both of which were suggested by DL as possible valid translations.
I wrote "A long time ago, surely!" but was marked WRONG.
But the sense of the French sentence here is something that won't be happening for a long time in the future, I think. Not sure how best to put it in English but I think the recommended answer "In a long time" is meant to mean "[That's not going to happen] for a long time [yet], surely!". You can't reduce the English sentence in exactly the same way as the French sentence has been reduced, though; if you had to give an equally short sentence in natural English it would probably be "A long way away, surely!"
I could envisage "[That's] a long way away, surely/to be sure!" coming after a discussion of some future hypothetical, say for example after a whole paragraph detailing a world with no war, inequality or hunger. Or something like "That's a long way off, of course."
Don't be too hasty. The primary definition of "sûrement" is "most probably" according to the Oxford French Dictionary, a resource which I trust. I.e., even though it looks like it would be "surely" and we want to make that mean "absolutely", it does not mean that at all. It means "most probably".
"A long time from now" would be a better English translation. "In [or "For"] a long time" most often refers to something that has not happened since a long time ago in the past, and so, using those words to refer to something that may happen in the future creates ambiguity.