In Bangkok, you're always likely to find a shop open somewhere, at any time of day. In the centre of the city, there are many 24 hour shops where you can buy basic products you need day to day. And the shops normally stay open until 8 pm at least, so you should have no problems there
Bangkok's monuments normally open until 6 pm, although some major tourist attractions such as the Royal Palace close beforehand, at 3:30 pm. We recommend that you check out the opening times of each monument individually.