You are free to keep earnings in your Ibotta account as long as you'd like, especially if you're saving up for something big! You may allow your earnings to accumulate until you reach your personal goal, then withdraw when the time is right for you.
However, when you keep your earnings in your account, the earnings may be subject to account maintenance fees if 180 days have passed without redeeming an offer or receiving credit for a referral bonus. To avoid account maintenance fee deductions, complete a redemption for any offer, or receive credit for a referral bonus at least once every 180 days.
For more about account maintenance fees, please check out this helpful article: What are Account Maintenance Fees?