Absent the USA in the war, with masses of US troops flooding in to France, would there even be a Macedonian Front or if there was one, how active would it be. You can't pull troops and resources away from the Western Front to go anywhere else unless you have somebody/something to fill the gaps. US combat forces even by November, 1918, were more often in quiet sectors training for trench warfare than in more active sectors. However every mile of "quiet" front occupied by US forces meant that many front line troops, as well as much of their support that could be rested, used elsewhere, and so forth. Similarly troops/assets replaced in French ports by US service troops could be moved to Salonika or elsewhere to perform those same vital functions. No US service, troops, no available French/British equivalents. If you thin out quiet fronts too much, they soon become active (the Germans are not idiots), if you reduce support troops in France, absent replacement support/supply is hampered.
It seems that evidence is that the Spring Offensives of 1918 were largely prompted by the German conclusion that they had to end things before the literally millions of fresh US forces were ready to go. No USA in the war means the main driver for that offensive is not there. Limited offensives, perhaps, but the bet everything on one roll of the dice, nope. The Germans will know about the French mutinies in summer, 1917, and without "the Americans are coming, stand fast" I expect they will be worse. Limited offensives to deal with salients, or seize specific areas likely, combined with straightening the lines and building up defenses to bleed the Entente even more.
The Entente here has severe financial issues (no unsecured US loans) and manpower issues as well as morale problems especially in the French Army. Because of the situation on the all important Western Front, and the collapse of Russia, while they can defend against any major German offensives (like OTL) they really can't do much offensively leaving Germany in control of key areas of France. As long as Germany and A-H are defensively oriented on the Italian Front, they bleed Italy and lose little, and can take advantage of weakness to be offensive at the right time. Italy is shaky. Germany is gaining resources from the conquered east/Brest-Litovsk gains, the issue is how much that helps on the home front. A-H is shaky but if they are defending against Italy, have no active Eastern Front, and Macedonia is absent or much reduced for reasons described, I expect they can last a good bit longer than OTL.
In sum, the question becomes a morale/political one. Which side sees collapse staring it in the face with political unrest. The CP have victories and basically a one front war now, food/supplies are probably getting better. The Entente has almost all of Belgium occupied, key parts of France occupied and stalemate in front of it with increasing financial pinch and no cavalry riding over the hill for succor. The collapse of the Ottomans, assuming it happens more or less as OTL, the occupation of German colonies are nice, but really not the sort of wins that will re-energize the populace. If the Germans are smart about it, they can start with a less than maximalist bargaining position, and settle for the east and perhaps minor gains in the west, and try and split the UK and France - after all, if Belgium is mostly restored, the UK keeps captured German colonies, they get out not so badly and Britain is not going to see German forces coming ashore... Of course if the Germans are stupid and make the sorts of demands they would in a Diktat, the war drags on longer.