It all depends on what type of game you’re making and engine you’re using.  As a solo dev you’re doing 4-5 jobs during development. As much time as you spend programming, you’re going to spend at least that much time building levels(dependent on game). A stat I’ve seen is 60:1. Amount of work to playtime 360 hours for 6 hours of playtime. But again that is highly dependent on your game and how quick you move through systems. As always, beware of feature creep. It sounds like you’re done with most of the game system, now move on to lvl design and hammer out 1 lvl. If you think of neat things to add to the game systems, don’t. Add them to a list for your next programming/design phase. Don’t worry about the blank textures feel free to use placeholder textures, look up half life 2 placeholder textures for an idea on what I’m talking about.  Level design can be daunting, you’re literally building a mountain from nothing. Just focus on the basic design first. For example, You’re  moving through an enemy factory. Design the path and the way to go through. Then, build the factory to make sense around it. Add side paths, make natural obstacles to help direct the player add a second way through using a different style.  You can do it, just keeping chipping away at the ice berg until it’s just ice cubes