Is it correct to use the phrase look forward to with the conjunction when? For example,
I am looking forward to when I can get my diploma.
Would it be more natural to use the word time instead? For example,
I am looking forward to the time I can get my diploma.
The grammar is OK in both sentences, but not great in style or semantics.
"I am looking forward to getting my diploma" is a better version of both your example sentences because it conveys the same meaning and feeling without any extra words. Shorter is better.
If you want to keep the longer structure, then semantically, "I am looking forward to the moment I get my diploma". Getting a diploma is a moment, rather than a time.
"Looking forward to a time (when)..." refers to an extended time, and usually one where we don't know when or even if it will happen:
I'm looking forward to a time when I don't have to worry about finding work, and I can buy things I need without checking my bank balance.
In sum, your example about receiving your diploma is simple, about a single moment, and it has a clear time when it will happen, so it doesn't work naturally with "looking forward to a time".
Your usage of when is common enough. Probably not going to be recommended by many authorities on style, but certainly not wrong.
