When do you replace your DE razor blade?

I've been using a safety razor for about 3 months and I'd like to know when you guys replace the blade. Because I'm starting to realize that the blade lasts me one shave. The second time I use it, the sharpness of the blade is gone.

I'm using the Merkur Mk34c closed comb
Astra blades
Brush with Proraso soap
Nivea aftershave

When I finish shaving I rub the blade with alcohol and put it back on the safety razor and let it dry there.

Any recommendation will be appreciated.