Why does it take very long to start downloading from the playstore? It typically take from 5 minutes to hours after I tap on download or update in playstore. Sometimes I can activate downloading process by opening up Google map or Google assistant.
You have not mentioned how you are connected to Playstore. Example Wifi, Broadband or Wifi through a Tether. Its too general a question.

The most common reason in this case is due to the network condition. Try to install at different time of the day. Switch to different networking environment and see the difference.