It depends upon how many 35mm slides you have.
And how much time you have.
It you have everything well organized, it will take about 2 to 3 minutes per slide with a 35mm slide scanner.
If you are using a flat bed 35mm scanner, the time will be about double that.
That time includes cleaning the slides.
I used compressed air on each slide to remove dust.
Yes, there will be dust on most of them.
You might consider having the scanning done by a professional.
Search the Internet for the professionals.
And you will want to touch up some of the images.
That time is in addition to the scanning time.
But I consider that fun time.