Bittrex updated their announcement with a specific time they are closing their ZCL wallets for maintenance:
https://bittrex.zendesk.com/hc/en-us/articles/115000201171-Support-for-the-ZEN-HardforkSupport for the ZEN Hardfork
Note: The estimated time of the fork block is now 8 hours earlier due to increased hash power on the network. We've also updated the time we are shutting down the market and wallet.
The ZenCash (ZEN) network is anticipated to launch on May 23rd at 5pm PDT (0:00 UTC) as a hard fork from Zclassic (ZCL). Bittrex users will be credited 1 ZEN for each Zclassic (ZCL) held in their exchange accounts at Zclassic block 110,000. The block is estimated to occur at approximately 11:10 PDT (18:10 UTC) 5/18/2017.
Bittrex will credit the ZEN tokens and enable the market when we and the ZEN team agree the new proof of work mining network is stable. The ZCL market will be re-enabled and all orders cancelled within 24 hours after the fork block.
Note: Bittrex will be closing our ZCL wallet and market within two hours of the fork block to take accurate snapshots of balances. If your deposits are not settled (e.g. marked as pending) by 09:00 PDT 5/18/2017, you may not be credited with ZEN.
Please note that the full 3 day gap between block 110,000 and the May 23rd launch will be used to generate and distribute the ZenCash genesis block, which importantly implements replay attack mitigation in the new client release.
We would like to thank the ZEN team for working closely with us and addressing our concerns. Distribution through hard forks like this one are not typically supported by Bittrex but strong replay protection and a commitment by both teams to maintain the ZCL and ZEN forks and their independent missions swayed our decision making.