Standby hotend and bed heating with inactivity timer after print?
Pausing
You could set temperatures for hotend and bed (respectively M104 and M140 and after that introduce a pause/dwell time with G4. After the pause, you could lower temperatures and proceed the shutdown of the printer, as such the setting of the idle temperatures and dwell time need to be done in the beginning of your end G-code in your slicer.
Not powering down
Alternatively, you could just not power down the hotend and bed. A typical end G-code consists of de-powering the fans, steppers, temperatures and positioning the head in a parking position, e.g.:

 M106 S0              ; Turn fan off
 M104 S0              ; Turn extruder off
 M140 S0              ; Turn off bed
// G91                  ; Change to relative positioning
// G1 Z30 E-2 F3000     ; Raise Z 30mm (lowers the bed) NEVER DO THIS, IT WILL DESTROY YOUR PRINTER IF YOU PRINT LARGER THAN MAX-Z MINUS 30 MM
// G90                  ; Switch back to absolute mode
 G1 X0 Y0             ; Move X/Y to origin
 M84                  ; Disable steppers

You could easily change the temperatures to standby values:

 M106 S0              ; Turn fan off
 M104 S160            ; Turn extruder to standby
 M140 S40             ; Turn bed to standby
// G91                  ; Change to relative positioning
// G1 Z30 E-2 F3000     ; Raise Z 30mm (lowers the bed) NEVER DO THIS, IT WILL DESTROY YOUR PRINTER IF YOU PRINT LARGER THAN MAX-Z MINUS 30 MM
// G90                  ; Switch back to absolute mode
 G1 X0 Y0             ; Move X/Y to origin
 M84                  ; Disable steppers