What would be your advice if we don’t plan to marry yet just visit each other from time to time?

You will need to apply for a fiance visa, and yes, you will have to get married within 90 days, or leave.

If it is your intention to visit in the U.S. and then return to your home in the Philippines (rather than an intention to enter the U.S. and then apply to remain permanently), then with that "nonimmigrant intent" you may apply for a visitor's visa for this purpose. If you were to have nonimmigrant intent but a reasonable time after you arrived in the U.S. you became engaged to be married and decided you would like to remain permanently, that may be satisfactory and then after your wedding you and your U.S. citizen spouse could proceed with the process of applying to become a Lawful Permanent Resident (getting a "Green Card"). If you were to enter the U.S. with a fiance visa, you would be expected to become married and file an adjustment of status application within 90 days, and if you failed to do so you would be regarded as having overstayed your visa and you would be at risk of being placed in removal/deportation proceedings. Nonetheless, if you were to become married later than 90 days after entry into the U.S., it still may be possible for you and your U.S. citizen spouse to proceed with an adjustment of status application process. Beyond these details there are many others that relate to eligibility.