Solar Year 453: Toran Liberation Army
Following the killing of her fiance Achilles, Odessa Silverberg formed the Toran Liberation Army, hatching numerous sabotaging plots against the Scarlet Moon Empire. Bartz, Fred Maximillian, and Watari were born.
Solar Year 454: Lilly Pendragon, Chris Lightfellow, Jacques, Muto, and Rhett were born.
Solar Year 455: Rebel Leader Killed
Tipped off by an informant, the Imperial Royal Guards attacked the Toran Liberation Army's hideout in Lenankamp. Odessa Silverberg was killed in this incident, crippling the Toran Liberation Army. Remnants of the Liberation Army regrouped at the Toran Lake Fortress with Tir McDohl as their leader and counterattacked against the Imperial Army, starting the Gate Rune Wars. Pilika, Makumaku, Mikumiku, Mukumuku, Mokumoku, Mekumeku, Shizu, Landis, and Wilder were born.
Solar Year 456: Jowstone Attacks Moravia
In order to defeat Kasim Hazil's forces in Moravia, Mathiu Silverberg sent a letter to the Jowstone Alliance, telling them the Scarlet Moon Empire was about to attack them. In response, Tinto and South Window sent their troops into Senan, diverting Kasim Hazil's forces toward them. Kasim Hazil soon surrendered to the Toran Liberation Army. However, Senan became occupied by Jowstone forces.
Solar Year 457: Gate Rune Wars Conclude
Tir McDohl led the Toran Liberation Army to victory after conquering the fortress of Shasarazade and the capital, Gregminster. Emperor Barbarossa disappeared and the Scarlet Moon Empire ended. The Toran Republic was created with Lepant as its first president. Edge, Gorou, Juan, Scott, Sarah (S3), and Nei were born.
So 455 is when McDohl took command of the Liberation Army. Depending on how long it took to get from being a little noble in Gregminster to there, and when these events took place within their respective years, Suikoden's story could cover from just over 1 year to perhaps 3.5 years. 1 year only works if the initial events are the waning days of 455, and most of the story actually occurs in 456, while the war ends in the first days of 457. In any case, calculating from the timeline doesn't tell us the duration of the game with any accuracy.