Depends on your strat, whether indy/techer/etc, and how early/late in the reset it is, and which server. This is very circumstantial and the answer will be different for every server for every strat, whether you are stocking, destocking or doing a startup.

Generally speaking, these 2 main factors will affect how often you login:
1. How long PSes take to return home. This is the main factor during grabbing phase. On express, you login every 2 hours and ignore the 12/18h bonus. On Team, every 12 hours. On Primary, every 20-24 hours. On Alliance, it depends whether you are WR-feeding (every 6 hours to sell goods), bottomfeeding (every 30 min to check for new farmland) or landtrading (once a day).

2. Whether you are stocking, market-playing, about to destock, at the start of the reset doing tech starts, etc. What is optimal is situational. For stocking, usually exactly every 18 hours (except Express). For market play, either 12 or 18. For destocking, either irrelevant for Theo jumps, or you might be doing a 40 hour logout(pri/all).