Learn English – Singular or plural (ensures)

"Series" can be singular or plural depending on context. Both of your sentences are therefore correct but different.

I like to watch TV series.

uses series as a plural and means you like to watch a number of different TV shows.

I like to watch a TV series.

uses series as singular and means you like to watch one TV show.

I'm a statistician. "Variance of waiting time" is how I would word this and how I have seen it worded elsewhere. This is similar to other uses of variance. We say the "variance of weight" or "variance of income" etc.

So, "of" and singular.