Bacteria Replication
Referring to the six stages in your diagram as Stage 1 through Stage 6, let's look at Stage 3: At that point, there is one bidirectional replication in progress, having made it about halfway around the chromosome. So it has 2 copies of the first half of the chromosome, and 1 copy of the rest, which is yet to be copied.  To get to this state of replication took 20 minutes.

Now the cell starts two new replications, one at "origin of replication" on each of the two copies of the first halves of partially replicated chromosome.  This is the state shown in Stage 4.  There are now three replications going on: the original, which is working on the last half of the chromosome, and two replications, which are working on (their own copy of) the first half of the chromosome.  All three replications will continue working for about 20 minutes.

We then get to Stage 5: the original replication has finished and the septum forms to separate the daughter cells.  The two replications started in stage 4 are continuing, and are about halfway down the chromosome.

The cells separate, and we are at Stage 6.  But stage 6 is the same state for each daughter cell as we had in Stage 3: a chromosomal replication that is halfway through (thus having one and a half copies).  It took 20 minutes to get from Stage 3 to Stage 6, where we are ready to double again.  During that 20 minutes, only one half of a chromosome had time to be copied, so it takes twice that (40 minutes) for replication of the whole chromosome.  This works out because the cell is doing three replications at once.