Assuming that you arrive on time and get through immigration quickly ... reaching the airport station by 10.15am, you can estimate one hour to get to Marienplatz, one hour to enjoy some sausage and beer, and one hour to get back. That puts you back at the terminal by 1.15pm, a comfortable 90 minutes before your departure.

I'd abandon the idea if I didn't make it to the airport station by 10.30am. Some people might be happy to cut things finer. Munich is a very efficient airport. (I recently did a 30 minute connection there - very impressive).