Check your credits anytime in My Games. You'll see:
Credits available: your current balance (for example 40/70)
Breakdown: your credits compared with your plan's total
Renewal countdown: when you get new credits (for example "Credits renew in 11 days")
Transaction history: tap View My History for everything you received and used
Plan credits expire at the end of each 4-week billing cycle, so use them before your renewal date.