They are free to travel and as I said, when they apply for indefinite leave to remain after 5 years, only absences of 6 months or more need to be declared. There is no official guidance. Form Set(M) and the Set(M) guidance notes is the best to look at for now.
See point 2.13, also Section 7 and also point 9.11 of the Set(M) form. This is the current form and is likely to change by the time your spouse reaches 5 years residence.
https://www.gov.uk/government/uploads/s ... _04-15.pdf
You will also need to prove with documentary evidence, that you and yous spouse have lived together (cohabitation) in the UK.
How long do you think they will be living out of the UK during the 5 years? A lot of absences will have an impact on their British Citizen application if they want to apply for it in a few years time.