Hi @srinivas s if you're using rack awareness you should probably get rid of 50 datanodes by decomissioning them without loosing some blocks, otherwise you'll probably will.
rebalance time depends on your network and cluster utilization, you can adjust some parameters to make it fastest if necessary, basically
hdfs dfsadmin -setBalancerBandwidth <bandwidth (kb/s)>
or within your HDFS params (example) :
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