eBay Bucks promos usually last two days, but they have gone on for three days; always on the weekdays and I don't think ever on the weekends.
@DIS_MERI has posted almost all the dates when an eBay Bucks promo has been announced for about the last six months to give you an idea of the frequency. My own vague experience seems to say they're most often announced on a Tuesday, but have been announced also on Wednesdays and Thursdays.
Three potential issues:
(1) eBay limits the total value of gift cards used in a transaction to $1000 so anything more, whether additional gift cards or shipping costs, would be paid for out-of-pocket. eBay's rules claim to limit the number of gift cards allowed per transaction to 8, but many here report being able to use more, like 14 or 15, but the $1000 value limit is more strictly enforced.
(2) When buying Target cards from Gift Card Mall on eBay, it is most cost efficient to buy the least number of the same denomination cards. GCM sells Target cards in $50/100/150 denominations. For example, for ~$1000, you should buy either 6x150 ($900) or 7x150 ($1050), rather than 10x100 ($1000) or 6x150 + 1x100 ($1000).
The reason is you need to think about minimizing your shipping costs, which eats into your profit. GCM charges $2.99 shipping for the first card of any denomination and $0.99 for each additional card of the same denomination.
So for the above examples, shipping charges would be $7.94 for 6x150, $8.93 for 7x150, $11.90 for 10x100, and $10.93 for 6x150 + 1x100. When spending Target cards, it's also to your benefit to have higher denomination cards because Target limits the number of gift cards per transaction to 4.
(3) As
@DIS_MERI pointed out while I was replying, if you're making this purchase during a 5x (10%) eBay Bucks promo, any subtotal more than $1000 will result in eBay Bucks exceeding the maximum $100 per transaction. So at $1000 and 10% back, you're fine to get $100 eBay Bucks; but at $1050 and 10% back, you're leaving $5 on the table so it's worth it to just pay the $2.99 shipping for another card in a new order. You'll max out eBay Bucks on a 4x bonus at $1250, and $1,666.66 on a 3x bonus.