It is very important to have an attorney to advise you and go with you and represent you before the immigration court. The requirements for voluntary departure depend on whether it is pre-hearing voluntary departure or at the conclusion of proceedings. It appears you are talking about pre-hearing, i.e. before you ask for any relief. Again, whether you should even ask for vol departure is a consideration in and of itself and you should have an AILA attorney go with you and counsel you before you go before the Judge and ask for this relief.
In answer to your question, you may be able to ask for 120 days (if pre-hearing) and finish up your community service during that time. Since it appears you have some type of criminal disposition it is not clear you will be granted VD which is a discretionary decision by the immigration judge in any event.
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I would still work with an attorney. VD is a very serious step to take and you have to make sure you understand the requirements, consequences and consequences of not complying.
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It seems you have a couple things going on in your life.
Based on what you have shared, I believe one question is whether time to complete community service is a sufficient basis to request a continuance of your hearing date? I would not suggest this as a basis to request a continuance as it does not directly pertain to your ability to have your immigration issue litigated.
Another possible issue is the effect of the criminal issue. This has at least two parts that could impact you from an immigration perspective. First, the nature of the crime may carry immigration consequences including permanent bar. Second, if not of a severity or nature to create a permanent bar, failing to complete your community service could result in the issuance of a warrant that will likely be a factor if you ever applied for another visa.
I strongly suggest that you retain an immigration attorney to assist you in properly navigating your issues.