One problem seems to be that 1 per year is too much in the long run, but not enough to keep on newbies. So, put a shorter interval that lengthens over time. There are lots of options for this.
How about assigning the number of active days for the next WP according to the Fibonacci series? Like 30 * (+1, +1, +2, +3, +5, +8, +13, +21, etc.)
30, 60, 120, 210, 360, 600, 990, 1620, etc.
Or a geometric progression. 30 * 3^x
30, 90, 270, 810, 2430, etc
Or, something else.
30 * x^2: 30, 120, 270, 480, 750, 1080, 1470, ...
10 * x^3: 10, 80, 270, 640, 1250, ...
30 * x! : 30, 60, 180, 720, 3600, ...
If 666 is a nice age for a WP (it's a point that is noticed quite a lot, so it makes sense), then several possibilities include:
8 x^4 + 2 x^2 : 10, 136, [b]666[/b], 2080, ...
24 x^3 + 2 x^2: 26, 200, [b]666[/b], 1568, ...
37x^2 *(x+1)/2: 37, 222, [b]666[/b], 1480, ...
for a 3rd WP
10 x^3 +x^2+2x+2: 15, 90, 287, [b]666[/b], 1287, ...
10 x^3 + 2x + 18: 30, 102, 294, [b]666[/b], 1278, ...
for a 4th WP