There are lots of variables in use, conditions etc so no one size fits all. I’m most cases you shouldn’t need to open the reel just to grease in between regular cleanings. Keeping the level wind brushed off and oiled, bearings oiled as needed and oiling accessible turning, sliding, moving parts should keep things working well. Occasionally pull the spool and swab off the rim and frame. If your reel has a port like Lews and ABUs adding some grease mid season or stretch the cleaning interval a little doesn’t hurt. Iabass8 is right, interim care is very important.