This is not a simple issue because its really up to the court how long they keep him while the Grand Jury decides his fate. I suggest you should consult with an attorney and see how much bail or bond will be necessary and then make your decision accordingly. If he's already in the service, he may still be subject to the military police picking him up so make sure to ask the attorney about that as well.
Yes. In Georgia, you have a right to a bond after 90 days if your case has not been indicted - although this period can be extended in some circumstances. That doesn't mean you will be able to afford the bond. The amount of the bond will be determined by the judge and that is difficult to predict. Armed Robbery bonds must be set by a Superior Court judge. An attorney can help you file the motion and present your husband's case for a bond to the judge.
I would expect the filing of a Motion for bond making these allegations would prompt both the presiding judge and the DA to move this case along. The pressure would be upon the DA to justify the danger and risks of a bond. Being in the military would almost always result in their being involved in his release.
Yes after 90 days if there has been no indictment a judge will generally grant a bond (with a few exceptions). The problem, as you suspected, is the nature of the charge is so serious it could result in a very large bond, and would likely have tons of conditions like leg monitor, house arrest, and so forth.