It totally depends on the size of the target web.
Typically, a serial web scraper will make requests in a loop, one after the other, with each request taking 2-3 seconds to complete.
This approach is fine if your crawler is only required to make <40,000 requests per day (a request every 2 seconds equals 43,200 requests per day). However, past this point, you will need to transition to a crawling architecture that will allow you to scrape millions of requests per day with no decrease in performance.
Therefore, you can get a sense of how long it will take by how many requests you will make and how long each request takes.