多会 is usually short for 多会儿 (duō huìr), a colloquial northern Chinese way of asking "when" — equivalent to 什么时候 (shénme shíhou). You'll hear it a lot in Beijing-area speech and in some other Mandarin dialects, but it's less common in writing or formal speech, and can sound quite regional or folksy if used elsewhere.

The 儿 (r) suffix is typical of Beijing retroflex speech (erhua), so in casual writing people sometimes drop it and just write 多会, though the pronunciation is still duōhuìr.

Example: 你多会儿回来?(Nǐ duōhuìr huílai?) — "When are you coming back?"

A close cousin is 多咱 (duō zán), which means the same thing and is even more dialectal, mostly heard in northeastern China.

If you're learning standard Mandarin for general use, 什么时候 is the safe, neutral choice. 多会儿 is good to recognize when you hear it, especially from Beijing speakers, but using 多咱 or writing 多会 without context might confuse learners of standard Mandarin.