Crown of Thorns help
My Crown of Thorns has not blossomed ever, it has been about five years. It is also very thin and spindly. It is indoors and under a window next to the register. I soak it through every couple weeks. I have attached pictures.
Certified GKH Gardening Expert
They are usually very forgiving but do need a lot of light - 3 to 4 hours of direct sun a day. That is probably why it hasn't bloomed and looks lanky. A south window gives the most light. You may need to add a grow light if you don't have a window with direct light. Being next to a register isn't good for a plant so I would move it for that reason, too.
Make sure it is getting regular fertilizer, too.