The timing is almost entirely a function of how busy your judge is and varies greatly even from court to court within the same judicial system, and from judge to judge within a particular court.
A month would be a completely unexceptional length of time to wait for a ruling in almost every U.S. jurisdiction (with some narrow exceptions for highly time sensitive matters like election law cases), even though the court has the authority to rule immediately once briefing is complete.
Indeed, in some very rare cases, I have seen a court deny a motion for summary judgment before the other party's response can be filed, and I have seen a court grant a motion for summary judgment before a reply is filed.
Where I practice, in Denver, Colorado, a motion for summary judgment in the state trial court of general jurisdiction is typically ruled upon in one to four months after the motion is fully briefed.
In Colorado, the state court administrator following formal notice from a party, can theoretically suspended a judge's salary if the judge takes more than two months from a motion being fully briefed to rule upon it. But these penalties are almost never invoked (to avoid making a lifetime enemy of the judge and because of the conventional wisdom that forcing the issue in this way is likely to cause a summary denial of the motion for summary judgment which is not an appealable order). I've never actually heard of a judge's salary actually being suspended as a result of this process, but it does impact judicial behavior and norms somewhat. I have actually seen it on rare occasions take more than a year for a judge to rule.
In the U.S. District Court for the District of Colorado, a ruling made within four months of the motion being fully briefed would be considered a swift ruling, it would be unexceptional for a ruling to take more than a year, and rulings taking more than two years after the motion is fully briefed are not unheard of.
In my experience, New York's state courts (where I am also admitted to practice) tend to rule on these motions two or three times as slowly as judges considering comparable motions do in Colorado.
Also, newly appointed judges tend to rule more promptly at first (partially because they don't have a backlog of old cases to deal with and partially out of enthusiasm for their new position and idealism), at least in areas of law with which they were familiar before becoming judges.
Newly appointed judges tend to be slower than average, however, in areas of law in which they did not practice before becoming judges (e.g. a criminal law practitioner who becomes a judge and then has to rule on a motion for summary judgment in an intellectual property dispute). I've even once seen a newly appointed judge confronted with a motion for summary judgment in a highly technical area of law where the judge had absolutely no background at all, appoint a special master sua sponte to recommend a ruling on the motion, which may have been faster in the long run, but which also delayed a ruling by the couple of months that it took to get the special master appointed.
Furthermore, for what it is worth, it tends to take the same amount of time as the time it takes to rule on a motion for summary judgement for a judge to rule on motions to dismiss under Rule 12, other motions that are in substance dispositive (like requests for class action certification or purely legal rulings on motions to compel arbitration), rulings on the merits following bench trials, and in state trial courts of general jurisdiction, rulings on appeals from lower courts or full briefed, purely legal writ petitions in the nature of a quasi-appeal.
Unopposed motions, or motions to which no timely response is filed, even if they are technically not motions for default judgment, tend to be ruled upon much more quickly.
The very rare motion for summary judgment or motion to dismiss that is not more than about five pages long that confines itself to a single simple issue (e.g. some motions to dismiss for lack of subject matter jurisdiction in federal court), also tend to be ruled upon more quickly.
Motions related to pro se habeas corpus petitions and other pro se prisoners' petitions in federal court have their own rhythm and timing, with which I am not familiar. In this kind of motion practice, most of the work is done by magistrates rather than full fledged judges.