UPDATE: 18 June 2015 - the merits from two weeks ago have now been halved properly, the merits from last week have not, so decay is confirmed to be 50% after the first week of earning the merits
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So Powerplay hit and even though I was excited, my enthusiasm got quickly deflated by the fact that you lose your rating with your power after a week and that your reputation with major faction decays while offline. Being a casual player who can only play after work, this is daunting.
Then I looked at what it takes to get to rating 5 in a power. Someone calculated that, based on trading, it would take 28.57 hours per day to achieve 10000 merits in one week. But it isn't the whole story because you do keep half your merits for the week after.
I found this post where calculations for what it actually takes were made.
To cut a long story short, if you are a casual player, you could farm 1300 merits a week and get to rating 5 in 39 weeks. Which is still quite a bit, but you can still achieve it. That is about 12,5 hours a week if you do it by trading. Once you reach the max number of 10000 merits, you will only need 1250 merits a week to keep your rating. When you look at that everything becomes a little more doable. Getting to max rank is quite the slog, but maintaining the rank may not be as bad as it first sounded, although the game still demands a certain amount of time from you each week. Providing you play little of anything else and always put that minimum amount of time in, if you are a worker like me you can probably maintain this.
There is a really good table that shows you the different levels of weekly merit income and how many weeks it would take you to achieve max rating. First number is the amount of merits you gain, second number is the amount of weeks it will take you to achieve rating 5 with that amount of income.
1300 = 39
1400 = 27
1500 = 22
1600 = 18
1700 = 16
1800 = 14
1900 = 13
2000 = 12
2100 = 11
2200 = 10
2300 = 9
2400 = 9
2500 = 8
2600 = 8
2700 = 7
2800 = 7
2900 = 7
3000 = 6
3100 = 6
3200 = 6
3300 = 6
3400 = 6
3500 = 5
3600 = 5
3700 = 5
3800 = 5
3900 = 5
4000 = 4
4100 = 4
4200 = 4
4300 = 4
4400 = 4
4500 = 4
4600 = 4
4700 = 4
4800 = 4
4900 = 4
5000 = 3
All the calculations are shown in the link I provided. I highlighted the thresholds of merits in yellow. They show you the most efficient number of merits/time, e.g., if you make 4000 merits a week, it will take you 4 weeks to get to rating 5. If you make 4900 merits a week, it will still take you 4 weeks, so don't bother making more than 4000 merits a week if you cannot make 5000. Same goes for making 3500 merits VS 3900 and so on.