Actually, the real time-issue with this stuff is recruitment and training. You can't forge a battle-ready army from a disparate collection of former Alliance soldiers and mercenaries in six months to a year.
Examples. The Byzantine Emperor Herakleios, who did not do this from a standing start,
took some five to six years to reorganize his armed forces, and that was only to create a relatively small, well-trained, quasi-commando force to fight in Adurbagadan. The United States military, also not from a standing start, required three and a half years (1940-1943) to assemble an army even
somewhat capable of undertaking the tasks given to it in the Second World War, and even then never, not even by 1945, hit the ultimate planned strength of two hundred divisions. Other attempts in a similar time frame, like the French Third Republic's desperate try to reassemble its military around the few Algerian divisions that had not been annihilated by the Prussians at Gravelotte and Sedan (in the 1870-1 Franco-Prussian War), were even more dismal failures.
Organizational difficulties with recruiting and training such a large force will not have dissipated over time and with technology. Estimates for training formations of soldiers have remained largely unchanged for modern armies by comparison with the armies of, say, the mid-nineteenth century. It does not suffice to simply run a soldier through the equivalents of Basic and AIT; at that point, a soldier is still raw, with little to no experience of actual military operations, combat or otherwise.
There are, of course, other concerns. For instance, these Cerberus "Firebases" around the galaxy need to be set up to provide a foundation for operations. This requires further time, albeit alleviated by the MEverse's extensive prefabricated construction technological advances. Still, it takes a great deal of time to actually figure out a decent location for these places, much less set up shop. When the US military constructed the immense logistical apparatus for Desert Storm in 1991, some six months were required, and that army was already experienced in developing logistical infrastructure and had the assistance of other armed forces - and, moreover, only had to do it in a relatively restricted area of
one planet. Cerberus has to do it all over the galaxy. Some of that work probably took place prior to the events of ME2, let alone Arrival, but did enough of it? I dunno.