I have a 75 AH deep cycle battery(AGM) and I have a battery charger with 2 modes, 5.5A for car, 2A for motorcycle, as far as I remember, lower charging rate is usually better for longer battery life and better charging, my questions are if what I remember is even true, and if so, does long charging affects the battery? For example, maybe lower charging rate might be better, but if it takes very long to charge it makes it worse? Long story short, which mode should I use, the 5.5A or the 2A? Thanks 
Maybe, but thats actually too much information, because in the charging post, they say that you need to charge the battery in 3 stages, “[1] constant-current charge, [2] topping charge and [3] float charge.” like here: 
But I have only 2 charging modes, 5.5A and 2A, which makes it not very helpful. What would you recommend?
Would need to know a lot more about your application to make a meaningful recommendation. If you intend to discharge/recharge frequently (daily) or only once in a while. If you are really concerned about battery life, you need a charger designed for the application, matched to the battery. Sounds like the charger you have is for occasional recharge of auto battery. Once a battery is charged its designed to be removed.
In general lower stress (voltage and current) on a battery is good.
I replied a while ago, and only now see that for a reason it did not add my reply. Anyway, the battery will not be used for daily use, more like a once a month, it is possible that I’ll use it twice or even a bit more times a month, and possible that I’ll use it only once in 2 months or so, so its not very accurate, but thats the use frequent more or less. The battery charger is a something my father is using for very long, and it should stop charging once the battery is full.
Your best bet to keep an infrequently used battery topped off and ready to go is a product called Battery Minder Plus - Northern Tool $29.99 on sale now. It will correctly float charge it and desulfate if needed. Use your car charger on 5 amps to bring the battery up from a deep discharge, then put the battery minder on it and let it float. Your battery should last a long time if you only discharge it once a month. I have alot of farm equipment that runs infrequently in the off season - I keep a Battery minder connected to all of them so they are ready when needed - some batteries are over ten years old.
Good Luck
What does “correctly float charge it and desulfate if needed” means? I don’t really understand what it actually does, can you please explain it?
Simply speaking, batteries are not perfect storage devices - they leak electrons!!! Left alone they will eventually discharge. A “float charge” is used to offset this “leaking” . Secondly, the internals of a battery will develop a sulfate coating - a by-product of the charge discharge process. or exposure to air. These sulfates impede the batteries charge discharge cycle. Sulfates can be reduced or removed by applying a special wave form to the battery periodically (Battery University has a more precise explanation). The Battery Minder can handle these task. But is not designed to restore a battery from a deep discharge in any reasonable amount of time - that is what your
dad’s charger does - but does not likely support float charging or de sulfating.
A little simplistic - read up on lead acid on the Battery University site!!!