Hi Niwantha, You need "a long time". You may also have heard of other time expressions such as "a short while", "..been a long while", "a long wait" and so on. In these expressions, "a" is needed. However, "while", "wait" and "time" are uncountable nouns in the context of time. For example, you do not say: "Last week, he waited for two long times." So, why we do use "a"? The best explanation I can give is that "long wait", "long while" and "long time" describe a long episode between some time in the past and the present. Example: Nice to see you after a long time. I last saw you three years ago. After that, we were each busy with our lives for three years (a long episode). Now, I see you again. I hope this helps.

"After a long time" will sound more natural to an English speaker.

Yes you need "a". Option b) Nice to see you after a long time. This option is better than a). The more natural way to say this is; "It has been a long time! Nice to see you". Alternatively; "Nice to see you! It has been a long time". In another situation; "It was nice to see (person) after a long time (away on holiday etc)". If you are speaking with the person you are seeing after a long time, it is better to break the sentence in two. If you are telling someone else about seeing someone, then the second expression works better. Adam T