Developers get paid within 45 days of the last day of the month. For example, the amount you earned in March should be paid to you before mid-May. For my case in March, 2015, I got paid on April 30.
Also note that the minimum threshold depends on the currency and country pair Apple set. If you decide to get paid in the pair Apple set (e.g. HKD in Hong Kong or JPY in Japan), it's $10 USD or equivalent. If not, it's $150 USD.
This information is from Apple's documentation in iTunes Connect.
When will I be paid?
Payments are made within 45 days of the last day of the month in which
book purchases were made. To receive payment, you must have provided
all required banking and tax information and documentation, as well as
meeting the minimum payment threshold.
If your bank and bank account currency is listed in this table, you
must exceed a minimum payment threshold of $10 USD. All other bank
countries and bank account currencies must exceed a minimum payment
threshold of $150 USD.