How long does it take for any changes that has been made on a certain Salesforce fields or records to get replicated by full extract in Einstein Analytics ? After changing a record value in the Salesforce object, I don't get them updated in the replicated dataset on running the replication on full extract.
If you want the data to be passed from the Salesforce objects to the Einstein Analytics datasets then you need to trigger the replication and/or dataflow runs.
If you don't trigger them manually or on a scheduled basis, there won't be any data passing to your Einstein Analytics dataset.
Enabling replication decouples the extraction of Salesforce data from your dataflows. If you have it disabled you need to run the dataflow directly, if you have it enabled then at first the replication, then the dataflow.
If you want to keep the changes in sync, you can define a more frequent scheduling of replication and/or dataflow runs.
By default, replication extracts data from each Salesforce object incrementally. Records are inserted, updated, or deleted to match changes in the object since the previous replication run. Because only changes are extracted, replication runs faster.
Naturally full extract takes longer than the incremental update but this also depends on the amount of data you have, if we are talking about hundreds of rows, then only few seconds will differ.
This link to 'Optimize Your Dataflows with Replication' should help you better understand the concept: https://sforce.co/2M7h1cI