Pipe welding is not that straight forward.  You have a number of welding passes to take into consideration, then NDT and finally hydrotesting.  I worked on a pipeline construction project in Iraq and used the following formulae to work out duration for the welding.

The average welder can complete about 140 inches of weld per hour in ordinary 1/4 inch wall line pipe.  Therefore, to find the output duration per day you would need to:

Number of welds per hour is equal to = ((140/((diameter of pipe - in inches)*3.14))).

This gives you an hourly rate of pipeline joints per hour. 

Times this by the number of hours in the shift to give the daily rate.

You can work out how many sections in the run by dividing the run by pipeline length (typically 12m). 

Then divide this answer by the answer to the welds per hour.  This will give you the outline duration for the welding. 

The welding runs normally follow each other so add half a day lag for each pass (1 day extra). 

Increase the overall duration by the estimated time to string the pipeline. 

NDT can happen while the welding is ongoing (obviously on different sections of the pipeline).  Add a day overlap from the welding finish to complete NDT. 

Depending upon the length, include a day to fill the pipeline for Hydrostatic Testing, a day to test and a day to drain.

Then you will need to add in a day for hook up. 

The welding duration will decrease if you have more than 1 team completing the task.

We included an in country overhead factor which reduced productivity by 40% to allow for the heat (it was anything upto 60 degrees in the desert in summer), they did a lot of work before the sun came up etc. 

Overall, this rule of thumb worked very well.  Obviously, you will be rounding up rather than rounding down.

Additional...

I am involved in a number of pipelines across India and when they deploy automatic welding they can cover 500m a day on 30inch pipe.

Its a bit slower when you are going over a mountain range.