Slightly off topic, but when I was living in Berlin and teaching, I had some students (married couple) who went to Vegas and when they came back, I asked how they liked the casinos.
They were surprised to hear from me that they could have gone into OTHER casinos, if they didn't have a room there! Thus, they never went to a single other casino the entire time there were there! Then again, in Berlin casinos (yes, they have quite a few small ones) you have to pay a cover charge to get in.
I get the same shocked response from many Californians when they drive into the parking garage at casinos and say to me, "What, free parking?!"
The vast majority of casinos on The Strip are gigantic - I like to say the MGM Grand alone is the size of Belgium.
So to answer your question:
No, there are never lines to get into a casino - unless it is New Year's Eve after the fireworks or the opening day of a new casino. Otherwise, just saunter in leisurely. Granted, table games can be crowded on Friday or Saturday nights, but normally you can always find a seat at a table with a brief scan around the room.