Hello Everyone I am trying to do an upgrade from v7.15.5 to v8.5.5 and the upgrader is taking forever does anyone know how long it could take.?

This depends on how large the content database is, the resources available for the database server (and to a lesser extent the web app) and whether or not the database is on the same machine as the web app, local network, or remote network. With variations in these parameters I've seen it take anywhere from a few minutes (small site, running on a powerful machine) to 14 hours (very large site, running on underpowered hardware with the database on a remote network).

This is the last thing I can see on the log and the instalation is still running: {"@t":"2023-11-16T13:12:54.2665599Z","@mt":"An error occurred during installation step {Step}","@l":"Error","@x":"System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Umbraco.Web.Install.InstallException: The database failed to upgrade. ERROR: The database configuration failed with the following message: A transport-level error has occurred when sending the request to the server. (provider: TCP Provider, error: 0 - An existing connection was forcibly closed by the remote host.)\

Do I recall you had to go to a 8.0.X release first or use the proworks migration package? Or it was really slow and somwtimes failed? https://www.proworks.com/blog/archive/how-to-upgrade-umbraco-version-7-to-version-8