whenever you increase the temporal baseline, phase decorrelation will increase. So you can either stick to 6 or 12 day intervals or lose coherence (depending on your study site).
To reduce this effect, time-series approaches can be used which combine the interferograms of subsequent image pairs (AB, BC, CD…) and those which have suitable baseline configuration.
You might have a look at pyrate which does exactly what you need. https://github.com/GeoscienceAustralia/PyRate
More advanced techniques are based on persistent scatterer interferometry: StaMPS - Detailled instructions