The code for the timeout is submitted in SERVER-52903.

The IDL definition to configure the timeout is here:  https://github.com/mongodb/mongo/blob/a102bde490e4f63a5558551f31e3c834edeb4dd6/src/mongo/db/repl/repl_server_parameters.idl#L426-L435

Most likely the timeout will be determined by experiments and beta deployment. It should be as large as needed to almost guarantee that the blocking phase will go through without timing out even when the server is overloaded, but it should not be too large, because it blocks reads/writes to the migrated database.