<p>We are developing a desktop application(visual basic 6.0).We have our own logging framework. What are good practices? When we have a web application, then we can control the level of logging. How to go about in a desktop app?</p>

Any answer needs to be considered based on your environment:
Do your business rules (or laws)
require
certain activities to be logged?
Is security an issue?
Do actions need to be traceable?
Is the size of the log file (or the amount of disk space) an issue?
Is your motivation to record activity, to debug, or both?
With those in mind:
Consider allowing the user to select the verbosity, or "logging level."
Log all program errors and significant user errors.
Log any activities that affect system configuration or operation.
Log the start and end of user sessions.
Log the start and end of the application.
Consider logging the first time a significant activity occurs.
Other suggestions:
Include timestamps, either in each log or at the beginning of a "group" of logs, as best appropriate for your application.
If you're logging to a file, consider rotating the log (
i.e.
closing one file and opening a new one) when it reaches a certain size or age.
If the application contains several modules, include the name of the module in each log.
If more than one person uses the application (shared computer?), log the user ID at the beginning of each session.
Assign log "levels" based on the severity (Error, Warning, Info, Debug).  The
```
syslog
```
specification defines 7 "standard" levels that serve as a good reference.
Ask your customer what they expect to see in the log.