The formula
laps needed = ⌈price ÷ Wins per lap⌉. Read both numbers in-game: the price from the Trails or Anime Rosters menu, the payout from a lap of your farmable zone ending at its Wins pad. Subtract your idle income first — away training and Auto Wins keep paying while you are logged off, and the official description frames both as crowd-driven, so the crowd you built is also your overnight earner.
Worked example with sample numbers (swap in yours): a purchase priced at 1,200 Wins with a pad paying 150 per lap is eight laps. If your Auto Wins banked 300 Wins overnight, that is 900 left — six laps. The prices in your client are the real inputs; the division never changes.