All of this is based on observations, primarily by users here who have shared their first hand experience.
Galactic Readiness Decay seems to have multiple rates, depending on your readiness score, or perhaps on the source of the score.
In general, it appears to decay at a rate of 0.15% per hour or around 1% per 8 hours, or 3% per day to a minimum of 50%.
There have been some reports of up to 6% per day, though I haven't seen this number echoed elsewhere.
If your GR is low, between say 50-60% then it appears to slow the decay rate to around 1% per day.
It takes very little to beef this back up, if it is causing a problem for you. For example you can play multiplayer, by yourself, and complete the Geth mission ... it's fairly easy to achieve a bronze or silver victory.
As @Edwin notes, if you simply start and exit a game, it will not count as an active day. You have to finish the game, which is why I noted Bronze or Silver being fairly achievable on some missions.
A lot of this is dependent on class. As a Vanguard for example, you'll probably never want to use an Assault Rifle - they're just too heavy, and as a Soldier, you'll usually give SMG's a pass - everything they can do an AR does better. Meanwhile, an Infiltrator gets lots of bonuses when using their trusty Sniper Rifle, evening if irresponsibly shooting it from the hip.
The second consideration is range: at very long ranges, a Sniper Rifle offers top notch accuracy. AR's and Pistols Can be pretty good. SMG's can be flaky though, and shotguns are nigh useless. By contrast, Shotgubs ate devastating at short range, but the loss of peripheral vision while looking down that sniper scope can get you killed.
The next consideration is the defenses of your opponents. Slow firing, high caliber weapons like Sniper Rifles and Pistols chew through Armor, but are weak against Biotic Barriers and Shields. Rapid fire weapons with a lot of projectiles, by contrast, like SMG's and Shotguns chew up Shields and Barriers quickly, but have trouble with armor. Assault Rufles are a bit of an odd duck as they're 'pretty good' at handling everything.
Additionally there are mods availiable for several weapon types that alter their capabilities. You can add Armor-penetration to an SMG or a shotgun, or a melee upgrade to a pistol or shotgun. Maybe a scope on your SMG to improve its mid-long range performance?
Finally, you may want to pair some weapons up with certain specialty ammo. For instance, SMG's are devestating in combination with Cryo or Incendiary Ammo as their high rate of fire gives them more opportunities to proc the special effects provided by these ammo types than, say, a slow firing shotgun.
Best Answer
X is not fast switch - it is "switch to previous weapon".
If you want to switch between Assault Rifles and Shotguns (or whatever), you just need to make sure that those are the last two weapon types you've actually used.