Many societies throughout history have had all the men practice fighting and required them to serve in time of war.
Many primitive tribes required all men to practice their fighting skills and turn out to fight in case of war. In medieval England and Scotland all men were required to practice archery. That is why the 15th century Scottish Parliament passed laws forbidding golf and other pastimes, to keep men practicing archery instead of playing golf and other sports.
And of course medieval nobles spent a lot of time practicing fighting skills.
In the British colonies in North America all men were required to own muskets and belong to the militia and practice drilling and to serve in case of Indian or foreign wars.
In modern Switzerland all men are conscripted into the army and after a period in the army have to belong to the reserves for decades, ready to serve in case of war.
In all of those cases - and many other cases - the men don't spend all of their time practicing and preparing for war, but spend the majority of their time in civilian pursuits keeping their societies running, and of course the women spent all of their time with civilian tasks and helped keep their society running.
And your Amazon society wouldn't have any men to help with civilian tasks to keep their society running. The women would have to do everything necessary themselves, which would put a limit on how much time they could spending drilling and training for war.
Of course little girls could become old enough to do light housework and other civilian tasks years before they might start training and practicing for war, and women over a certain age limit might be freed from military obligations and able to do light tasks and supervise the girls. But strong young women would have to do all the hard physical labor which would reduce the proportion of the time they could spend on military training.
Unless the Amazon society isn't populated entirely by the Amazons. The Amazons could be the master class and there could be slaves or serfs or second, third, fourth, etc. class citizens below them.
Possibly the Amazons buy slaves - men, women, boys, & girls - and make them do all the civilian tasks. It might be possible for the slaves to gain their freedom and probably leave the island and seek their original homes. Note that slaves have rarely reproduced faster than they died and thus most slave owning societies have had to constantly import more slaves.
They might have a permanent class of serfs.
Possibly girls from neighboring countries serve terms as indentured servants on the Amazon island. They might be hired about age 12 to 15 and serve for ten years until about age 22 to 25 and then be sent back to their homes with payment based on the value of their contribution to Amazon society, payment which makes them attractive potential wives. Maybe a smaller number of men are also hired as indentured servants for the heavy work.
And in modern times the Amazons might hire foreign guest workers like wealthy Arab countries do and treat them harshly but much better than slaves.
Since the Amazon island is an island, the Amazons probably have a navy and train for naval fighting as well as land fighting.
And the Amazon island probably has many defensive fortifications, so that an invading enemy force would have to capture many fortifications with a heavy loss of life.
With modern weapons, physical strength is less important than it used to be. Thus many rebels and guerrillas use teenage and younger kids as combatants. So it is possible that every Amazon girl over the age of ten has a light weight assault rife and ammo and is trained how to use it.