Static imaging. How many 20sec exposures before realigning?

[Skyballer]

I'm doing 20 sec exposures of Lyra right now, first time with this samyang 14mm lens, new in general though. 

How many of these exposures would it be recommended to run before having to stop and re align vega back in the centre of my view? 

I know deep sky stacker will align the images so hoping I don't have to re align every 2 min? 

Thanks 

[jambouk]

It depends what you are trying to achieve. Vega will appears to move after about 30 seconds, but at the pixel level.