how do I retrieve a value after using setx?

from the doc (setx /? )

Because SETX writes variables to the master environment in the registry, edits will only take effect when a new command window is opened - they do not affect the current CMD or PowerShell session.

After using setx, you don't need to wait to reboot or get in a new instance/session to be able to get/use this value. this value can be read in Windows Register:

In your case:

setx TIME_VAR %time%

for /f "tokens=3 delims=^ " %%i in ('reg query HKCU\Environment ^| findstr /i /c:"TIME_VAR"') do  echo/%%i

setx TIME_VAR %time%

for /f "tokens=3 delims=^ " %%i in ('reg query HKCU\Environment ^| findstr /i /c:"TIME_VAR"') do  (

echo/ local and next session  = = = =  
echo/ Setx Reg Value = "%%i" 

echo/ local and next session  = = = =  
echo/ TIME_VAR  Value = %%i

:eof

result:

 :: local and next session  = = = = 
 Setx Reg Value = 13:32:05,15

 :: in same session and also in next
 TIME_VAR Value = 13:32:05,15