I've been in the CG Reserve for 15 years and an IV for the past 11 years, so I can answer your questions:
DEPOT is three weeks. It's specifically designed for Reservists who are enlisting on the basis of their civilian job skill set, or prior service who are enlisting in the active duty CG. As to what actually goes on there, I can't say, as DEPOT did not exist when I came in.
The CGIS FLETC course is two weeks at Brunswick. As of now, there's no schedule for when the next course will convine.
There are only three CWO billets in the Reserve side of CGIS, in effect this really means there's no CWO path. You'll advance in the IV rating. Advancement from IV3 through IV1 is not too difficult. Getting to Chief and beyond will be challenging.
As for volunteering for active duty periods, it's based strictly on unit needs, and funding. Those opportunities may never present themselves. You may spend your entire career doing your two days a month and two weeks a year. I've been fortuante. I've been on active duty assignments three times in my CGIS career (five times in my entire CGR career). 2006-07 for one year; 2010 for 60 days; 2015 for 30 days.
Occasionally, CGIS offices in other parts of the country may need bodies, and you can volunteer for those. That would mean leaving your family and civilian occupation for some period of time, so you need to make sure everyone's good with that.
If you have any further questions, feel free to PM me here.
**Mods please move to the Reserve or Ratings sub forum**
Thanks Chief. Moved to correct area.
Old Guard2 Macie