Learn English – correct? “Helps me get ready” or “Helps me getting ready”

Your first example is idiomatic; your second is not.

"Late getting back" is an elision of "late in getting back," where the gerund is used as complement to the preposition. Here, "in" carries a meaning something like "related to."

I was late in realizing that he was secretly a werewolf.

"To" does not have this meaning. The preposition "to" is used with "late" only where the complement is a noun:

I was late to the party.

I was late to my own wedding.

or with the adverb "too" to indicate that the lateness was excessive:

I was too late to save her.

He arrived too late to meet the famous molecatcher.

"So" does not work in these sentences the way that "too" does, so your second example would not be idiomatic in most English dialects.

Spoken English can have grammar that is not like traditional grammar for written texts or formal spoken texts. Often, in spoken English, their is truncation or shortening:

How long [will it be] before that thing is ready? becomes just leaves out the first verb. It is perfectly acceptable.

Also, "to get ready" is for people: He got ready for work early in the morning. "to be ready" is for people or things. For things, to be ready might apply to having some object repaired: How long [will it be] before that thing [some object] is ready.