What is the Always On Sync sync frequency? On an Always On Availability group, I understand an async secondary replica asks the primary for the blocks to be shipped. However, I would like to know how frequent is this task (to know the possible time frame of data loss) and to know if this frequency can be altered (to minimize the data the time frame of data loss).
Database replication does not occur on a specific timeframe--the primary replicates data to each secondary constantly.  If the secondary is asynchronous, the primary will replicate data to it as fast as it can.  If there are no bottlenecks, an asynchronous secondary will only be behind by what is currently in flight.

There are DMVs that provide statistics showing how much data is queued for each secondary and the last commit times.  There is a log send queue, which is basically transactions in the log on the primary that haven't yet been sent the secondary.  The other queue is the redo queue on the secondary, which is transactions that have been received and written to the log, but haven't yet been committed to the data file.

There are a number of articles, such as Measuring Availability Group synchronization lag, that have nice queries for quickly seeing how much data is in each of the queues, and any good monitoring software will also provide a view with this information.

Also, to be clear, your reference to "blocks" is incorrect as the database replication does not work on a block basis.  Database replication uses transactions.

So to answer your question about what you can do to minimize the amount of potential data loss, you simply have to make sure that the secondary has adequate storage performance to keep up, and that there is adequate network throughput to transfer data from the primary to the secondary as fast as it is generated.

One item to note is that bandwidth is not equal to trhoughput.  If you have a 100 Mbs link, you will not get 100 Mbs throughput.  If there is significant latency, your throughput will be much less. See  Why your maximum throughput is less than your bandwidth for a good explanation. WAN accelerators can be used if needed to overcome high latency on a link and provide better throughput.