Its mainly six weeks in Jan/Feb, and then four weeks in May think. Thats when they don't go across the river. Go 2-3 times a week, but won't be able to then. No idea how to get a bike across either, in January there won't even be any ferries.
I've checked the Merseyrail schedule for this and think it's worth summarising here.
Phase 1 - 3 January to 12 FebruaryEntire Loop line is closed. Wirral line trains terminating at Birkenhead North and Birkenhead Central. New Brighton/West Kirby trains will be six car, operating every 30 minutes.
Trains to Ellesmere Port will require you to change at Hooton during the day.
With rail replacement bus services running from Liverpool, expect to add at LEAST 20 minutes to your journey.
Phase 2 - 13 February to 29 MayMersey rail tunnel reopens, trains terminate at James Street. Restrictions re trains to New Brighton, West Kirby and Ellesmere Port above still apply.
There will be NO cross river trains at weekends, except for the Grand National Festival, Easter and May bank holidays. Trains will terminate as in Phase 1 aboveIn all cases where Hamilton Square is closed, rail replacement buses will link Birkenhead Central to Birkenhead North.
Phase 3 - 30 May to 18 JuneLoop line closes again completely.
As in Phase 1, trains will terminate at Birkenhead North and Birkenhead Central respectively. The same restrictions on Ellesmere Port trains applies.