Montana is a "no fault" state, which means the petitioner does not have to proof any kind of fault or misbehavior of respondent. The grounds here are "irreconcilable differences" or being separated 180 days. Once the petitioner satisfies the residency requirement, s/he can file.
The dissolution cannot be completed before certain time limits are met. For instance, after the respondent is properly served with process, respondent has 20 days to file an answer. When one party files a motion, the other party has 10 business days to respond. When all issues are resolved, the parties might have to wait two weeks for a hearing date.
Some dissolutions go very smoothly and quickly, others literally can take years. Six months is not outrageous when you compare it to cases where the parties fight for five years.