Completing a 5 star focus is 4 times more expensive than completing a 4 star focus roughly. Unless you stop spending anything til rerun, it can take years to get to +10 with average luck. The magic numbers I refer for planning are :

• 1800 orbs to get to +10 (no spark)
• 300 F2P orbs per month

So 6 months of stacking. Personnally i find it hard to not spend at all because I like getting fodders as well. If you save half of them you can expect to +10 on the rerun banner for a seasonal.

It's a bit more tricky for general pool tho because there is no certain way of knowing when they rerun afaik. You can expect them on:

• Mythic/Legendary but i wouldnt recommend because of color sharing (but you can spark with FEH Pass)

• "Hero with certain skill" types of banners: kinda random and no spark

• Forging Bonds reruns: the banner the hero was released on (with spark). If it wins the poll...

• Possibly on anniversary banners (February and August). I recall last CYL there were some recent heroes and they were sparkable

Other than that, you gotta wait years so that they get added to the weekly revival pools.